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

An optical modulator includes optical waveguides on which phase modulation regions are formed. A drive circuit includes a lower-bit drive unit, an upper-bit drive unit, and a bit splitting unit. The bit splitting unit splits an input digital signal into upper bits and lower bits. The lower-bit drive unit outputs a value obtained by performing D/A conversion on the lower bits to phase modulation regions. The upper-bit drive unit outputs, to phase modulation regions, a value greater than a maximum value of values output from the lower-bit drive unit, or a minimum value of the values output from the lower-bit drive unit, according to a value of the upper bits.

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 voof the modulated optical signal and a phase velocity vmof 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.

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 transmitter500having a typical segmented electrode structure will be described. The optical transmitter500is a multilevel modulation optical transmitter. In this case, however, to simplify the explanation, the optical transmitter500will be described as a 4-bit optical transmitter.FIG. 1is a block diagram schematically showing the configuration of the multilevel optical transmitter500having a typical segmented electrode structure. The optical transmitter500includes an optical modulator51, a decoder52, and a drive circuit53.

The optical modulator51outputs output light OUT which is obtained by modulating input light IN. The optical modulator51includes optical waveguides511and512, optical multiplexers/demultiplexers513and514, and phase modulation regions PM51_1to PM51_4and PM52_1to PM52_4. The optical waveguides511and512are arranged in parallel.

The optical multiplexer/demultiplexer153is disposed at the optical signal input (input light IN) side of the optical waveguides511and512. At the input side of the optical multiplexer/demultiplexer513, the input light IN is input to an input port P1, and an input port P2has no input. At the output side of the optical multiplexer/demultiplexer513, the optical waveguide511is connected to an output port P3and the optical waveguide512is connected to an output port P4.

FIG. 2Ais a diagram schematically showing the configuration of the optical multiplexer/demultiplexer513. In the optical multiplexer/demultiplexer513, the light which has entered the input port P propagates to the output ports P3and P4; however, the light propagating from the input port P1to the output port P4has a phase that is delayed by 90° relative to the light propagating from the input port P1to the output port P3. The light which has entered the input port P2propagates to the output ports P3and P4; however, the light propagating from the input port P2to the output port P3has a phase that is delayed by 90° relative to the light propagating from the input port P2to the output port P4.

The optical multiplexer/demultiplexer514is disposed at the optical signal output (output light OUT) side of the optical waveguides511and512. At the input side of the optical multiplexer/demultiplexer514, the optical waveguide511is connected to an input port P5and the optical waveguide512is connected to an input port P6. At the output side of the optical multiplexer/demultiplexer514, the output light OUT is output from an output port P7.

FIG. 2Bis a diagram schematically showing the configuration of the optical multiplexer/demultiplexer514. The optical multiplexer/demultiplexer514has a configuration similar to that of the optical multiplexer/demultiplexer513. The input ports P5and P6respectively correspond to the input ports P1and P2of the optical multiplexer/demultiplexer513. The output ports P7and P8respectively correspond to the output ports P3and P4of the optical multiplexer/demultiplexer513. The light which has entered the input port P5propagates to the output ports P7and P8; however, the light propagating from the input port P5to the output port P8has a phase that is delayed by 90° relative to the light propagating from the input port P5to the output port P7. The light which has entered the input port P6propagates to the output ports P7and P8; however, the light propagating from the input port P6to the output port P7has a phase that is delayed by 90° relative to the light propagating from the input port P6to the output port P8.

The phase modulation regions PM51_1to PM51_4are arranged on the optical waveguide511between the optical multiplexer/demultiplexer513and the optical multiplexer/demultiplexer514. The phase modulation regions PM52_1to PM52_4are arranged on the optical waveguide512between the optical multiplexer/demultiplexer513and the optical multiplexer/demultiplexer514.

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 waveguides511and512. That is, the optical modulator51forms a multilevel Mach-Zehnder optical modulator having two arms and an electrode segmented structure.

The decoder52decodes 4-bit input digital signals D[3:0], and outputs, for example, multi-bit signals D1to D4, to the drive circuit53.

The drive circuit53includes five-value D/A converters DAC51to DAC54. The D/A converters DAC51to DAC54are respectively supplied with the signals D1to D4. The D/A converters DAC51to DAC54output a pair of differential output signals according to the signals D1to D4, respectively. At this time, the positive-phase output signals of the differential output signals output from the D/A converters DAC51to DAC54are respectively output to the phase modulation regions PM51_1to PM51_4. The negative-phase output signals of the differential output signals output from the D/A converters DAC51to DAC54are respectively output to the phase modulation regions PM52_1to PM52_4.

The differential output signals output from the D/A converters DAC51to DAC54will now be described. As mentioned above, the D/A converter DAC51is a D/A converter which outputs five values (0, 1, 2, 3, and 4). Specifically, the DAC51increases 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 D1.

On the other hand, the DAC51outputs an inverted signal of the positive-phase output signal as the negative-phase output signal. Specifically, the DAC51increases 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 D1. 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. 3is an operation table showing operations of the optical transmitter500. 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 DAC51increases 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 DAC51is “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 DAC52increases 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 DAC52is “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 DAC52is “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 DAC53increases 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 DAC53is “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 DAC53is “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 DAC54increases 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 DAC51is “0” and the value of the negative-phase output signal is “4”.

The phase modulation operation of the optical transmitter500will now be described.FIG. 4is a diagram schematically showing a mode in which light propagates in the optical transmitter500. In this example, as shown inFIG. 1, the input light IN is input to the input port P1of the optical multiplexer/demultiplexer513. Accordingly, the light output from the output port P4has a phase that is delayed by 90° relative to the light output from the output port P3. After that, the light output from the output port P3passes through the phase modulation regions PM51_1to PM51_4and reaches the input port P5of the optical multiplexer/demultiplexer514. The light which has reached the input port P5directly reaches the output port P7. On the other hand, the light output from the output port P4passes through the phase modulation regions PM52_1to PM52_4and reaches the input port P6of the optical multiplexer/demultiplexer514. The light which has reached the input port P6has a phase that is further delayed by 90°, and reaches the output port P7.

In other words, light L2which reaches the output port P7from the input port P6has a phase that is delayed by 180° relative to light L1which reaches the output port P7from the input port P5, even when the phase modulation regions PM51_1to PM51_4and the phase modulation regions PM52_1to PM52_4do not perform any phase modulation.

FIG. 5Ais a constellation diagram showing the light L and light L2which are not subjected to phase modulation by the phase modulations regions PM51_1to PM51_4and the phase modulation regions PM52_1to PM52_4. As described above, the light L2which reaches the output port P7from the input port P6is delayed by 180° relative to the light L which reaches the output port P7from the input port P5.

Meanwhile, in the optical transmitter500, the positive-phase output signal is input to each of the phase modulation regions PM51_1to PM51_4, and the negative-phase output signal is input to each of the phase modulation regions PM52_1to PM52_4. Accordingly, the phase delay of the light L2which reaches the output port P7from the input port P6is compensated.FIG. 5Bis a constellation diagram showing the light L1and light L2when the binary code of the input digital signals D[3:0] is “0000” in the optical transmitter500. 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 PM51_1to PM51_4, and the negative-phase output signal indicating “4” is input to each of the phase modulation regions PM52_1to PM52_4. Accordingly, the phase of the light passing through the phase modulation regions PM52_1to PM52_4is further delayed by 180°.

That is, the phase delay of 180° generated due to the phase modulation regions PM52_1to PM52_4, as well as the original phase delay of 180°, is added to the light L2which reaches the output port P7from the input port P6. Thus, a phase delay of 360° is generated in the light L2which reaches the output port P7from the input port P6, so that the phase delay with respect to the light L1, which reaches the output port P7from the input port P5, 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 DAC51to DAC54increases.

FIG. 5Cis a constellation diagram showing the light L1and light L2in the optical transmitter500. As shown inFIG. 5C, when the differential output signals are used, the optical phases of the light L1and the light L2change symmetrically with respect to an Re axis, while the phase delay of the light L2, which reaches the output port P4from the input port P1and reaches the output port P7from the input port P6, 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 L1can be changed in 16 levels, i.e., 0 to 15 Δθ, and the amount of phase modulation of the light L2can 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 ofFIG. 3.

To facilitate understanding of the drawings,FIGS. 5B and 5Cillustrate the positions of the light L1and light L2so 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 L1and light L2may 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 L1and light L2, which are subjected to phase modulation by the drive circuit53, are at regular intervals, the following problem arises.FIG. 5Dis a constellation diagram showing the light intensity of the output light OUT obtained by multiplexing the light L1and light L2in the optical transmitter500. As shown inFIG. 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 transmitter100according to a first exemplary embodiment of the present invention will be described. The optical transmitter100is a multilevel modulation optical transmitter. In this case, however, to simplify the explanation, the optical transmitter100will be described as a 4-bit optical transmitter.FIG. 6is a block diagram schematically showing the configuration of the optical transmitter100according to the first exemplary embodiment. The optical transmitter100includes an optical modulator11and a drive circuit12.

The optical modulator11outputs output light OUT which is obtained by modulating input light IN. The optical modulator11includes optical waveguides111and112, optical multiplexers/demultiplexers113and114, and phase modulation regions PM11_1to PM11_4and PM12_1to PM12_4. The optical waveguides111and112are arranged in parallel.

The optical multiplexer/demultiplexer113is disposed at the optical signal input (input light IN) side of the optical waveguides111and112. The optical multiplexer/demultiplexer113has a configuration similar to that of the optical multiplexer/demultiplexer513described above. At the input side of the optical multiplexer/demultiplexer113, the input light IN is input to the input port P1, and the input port P2has no input. At the output side of the optical multiplexer/demultiplexer113, the optical waveguide Ill is connected to the output port P3and the optical waveguide112is connected to the output port P4.

The optical multiplexer/demultiplexer114is disposed at the optical signal output (output light OUT) side of the optical waveguides111and112. The optical multiplexer/demultiplexer114has a configuration similar to that of the optical multiplexer/demultiplexer514described above. At the input side of the optical multiplexer/demultiplexer114, the optical waveguide111is connected to the input port P5, and the optical waveguide112is connected to the input port P6. At the output side of the optical multiplexer/demultiplexer114, the output light OUT is output from the output port P7.

The phase modulation regions PM11_1to PM11_4are arranged on the optical waveguide111between the optical multiplexer/demultiplexer113and the optical multiplexer/demultiplexer114. The phase modulation regions PM12_1to PM12_4are arranged on the optical waveguide112between the optical multiplexer/demultiplexer113and the optical multiplexer/demultiplexer114.

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 waveguides111and112. That is, the optical modulator11forms a multilevel Mach-Zehnder optical modulator having two arms and an electrode segmented structure.

The drive circuit12includes a lower-bit drive unit121, an upper-bit drive unit122, and a bit splitting unit123. The bit splitting unit123splits the 4-bit input digital signals D[3:0], which are supplied to the drive circuit12, into upper bits and lower bits. In this case, the bit splitting unit123splits the input digital signals D[3:0] into two upper bits D[3:2] and two lower bits D[1:0].

FIG. 7is an operation table showing operations of the optical transmitter100according to the first exemplary embodiment. The lower-bit drive unit121is supplied with the lower bits D[1:0]. The lower-bit drive unit121outputs 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 unit121is output to the phase modulation region PM11_1. The negative-phase output signal of the differential output signals output from the lower-bit drive unit121is output to the phase modulation region PM12_1.

Specifically, the lower-bit drive unit121outputs four values (0, 1, 2, and 3) according to the lower bits D[1:0]. The lower-bit drive unit121increases 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 unit121outputs an inverted signal of the positive-phase output signal as the negative-phase output signal. Specifically, the lower-bit drive unit121decreases 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 unit122is supplied with the upper bits D[3:2]. The upper-bit drive unit122outputs 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 unit122are respectively output to the phase modulation regions PM11_2to PM11_4. The negative-phase output signals of the differential output signals output from the upper-bit drive unit122are respectively output to the phase modulation regions PM12_2to PM12_4. In the upper-bit drive unit122, 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 unit122outputs “0” as the positive-phase output signals to the phase modulation regions PM11_2to PM11_4and outputs “4” as the negative-phase output signals to the phase modulation regions PM12_2to PM12_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 unit122outputs “4” as the positive-phase output signal to the phase modulation region PM11_2and outputs “0” as the positive-phase output signals to the phase modulation regions PM11_3and PM11_4. Further, the upper-bit drive unit122outputs “0” as the negative-phase output signal to the phase modulation region PM12_2and outputs “4” as the negative-phase output signals to the phase modulation regions PM12_3and PM12_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 unit122outputs “4” as the positive-phase output signals to phase modulation regions PM11_2and PM11_3and outputs “0” as the positive-phase output signal to the phase modulation region PM11_4. The upper-bit drive unit122outputs “0” as the negative-phase output signals to the phase modulation regions PM12_2and PM12_3and outputs “4” as the negative-phase output signal to the phase modulation region PM12_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 unit122outputs “4” as the positive-phase output signals to the phase modulation regions PM11_2to PM11_4and outputs “0” as the negative-phase output signals to the phase modulation regions PM12_2to PM12_4.

That is, the upper-bit drive unit122performs a rough control according to the upper bits, whereas the lower-bit drive unit121performs 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 transmitter100is 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 transmitter200according to a second exemplary embodiment of the present invention will be described. The optical transmitter200is a specific example of the optical transmitter100according to the first exemplary embodiment.FIG. 8is a block diagram schematically showing the configuration of the optical transmitter200according to the second exemplary embodiment.

The lower-bit drive unit121includes a four-value D/A converter DAC1which is supplied with the lower bits D[1:0]. As shown inFIG. 7, the D/A converter DAC1outputs a positive-phase output signal to the phase modulation region PM11_1and outputs a negative-phase output signal to the phase modulation region PM12_1according to the lower bits D[1:0].

The upper-bit drive unit122includes a decoding unit21and binary drivers DRV1to DRV3. The decoding unit21converts the upper bits D[3:2] from a binary code to a thermometer code. The decoding unit21sequentially drives the drivers DRV1to DRV3in accordance with an increase in the thermometer code. The drivers DRV1to DRV3output 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 DRV1to DRV3are respectively output to the phase modulation regions PM11_2to PM11_4, and the negative-phase output signals are respectively output to the phase modulation regions PM12_2to PM12_4. The positive-phase output signal and negative-phase output signal output from each of the drivers DRV1to DRV3take 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 DRV1to DRV3output “0” as the positive-phase output signals to the phase modulation regions PM11_2to PM11_4, respectively. The drivers DRV1to DRV3output “4” as the negative-phase output signals to the phase modulation regions PM12_2to PM12_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 DRV1outputs “4” as the positive-phase output signal to the phase modulation region PM11_2, and outputs “0” as the negative-phase output signal to the phase modulation region PM12_2. The drivers DRV2and DRV3output “0” as the positive-phase output signals to the phase modulation regions PM11_3and PM11_4, respectively, and output “4” as the negative-phase output signals to the phase modulation regions PM12_3and PM12_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 DRV1and DRV2output “4” as the positive-phase output signals to the phase modulation regions PM11_2and PM11_3, respectively, and output “0” as the negative-phase output signals to the phase modulation regions PM12_2and PM12_3, respectively. The driver DRV3outputs “0” as the positive-phase output signal to the phase modulation region PM11_4and outputs “4” as the negative-phase output signal to the phase modulation region PM12_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 DRV1to DRV3output “4” as the positive-phase output signals to the phase modulation regions PM11_2to PM11_4, respectively. The drivers DRV1to DRV3output “0” as the negative-phase output signals to the phase modulation regions PM12_2to PM12_4, respectively.

Therefore, according to this configuration, the optical transmitter capable of performing an operation similar to that of the optical transmitter100according 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 DAC1of the lower-bit drive unit outputs 2n-level signals (“0” to “2n−1”) which are obtained by performing D/A conversion on an n-bit signal.

The upper-bit drive unit includes (2m−1) drivers. The (2n−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 (2m−1) drivers output “0” when the value of the m bits is 0. Among the (2m−1) drivers, the number of drivers that output “2n”, which is greater by 1 than the maximum value “2n−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 transmitter300according to a third exemplary embodiment of the present invention will be described. The optical transmitter300is a modified example of the optical transmitter100according to the first exemplary embodiment and the optical transmitter200according to the second exemplary embodiment.FIG. 9is a block diagram schematically showing the configuration of the optical transmitter300according to the third exemplary embodiment. The optical transmitter300includes an optical modulator31and a drive circuit32. The optical modulator31and the drive circuit32respectively correspond to the optical modulator11and the drive circuit12of the optical transmitters100and200. The optical transmitter300is configured as a 5-bit optical transmitter.

The optical modulator31includes the optical waveguides111and112, the optical multiplexers/demultiplexers113and114, and phase modulation regions PM31_1to PM31_3and PM32_1to32_3. The phase modulation regions PM31_1to PM31_3are arranged on the optical waveguide11between the optical multiplexer/demultiplexer113and the optical multiplexer/demultiplexer114. The phase modulation regions PM32_1to PM32_3are arranged on the optical waveguide112between the optical multiplexer/demultiplexer113and the optical multiplexer/demultiplexer114. The other components of the optical modulator31are similar to those of the optical modulator11, and so the description thereof is omitted.

The drive circuit32includes the lower-bit drive unit121, an upper-bit drive unit322, and a bit splitting unit323. The bit splitting unit323splits 5-bit input digital signals D[4:0], which are supplied to the drive circuit32, into upper bits and lower bits. In this case, the bit splitting unit323splits 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 unit121is similar to that of the second exemplary embodiment, and so the description thereof is omitted.

The upper-bit drive unit322is supplied with the upper bits D[4:2]. The upper-bit drive unit322includes a bit splitting unit324, a four-value D/A converter DAC2, and a binary driver DRV4. The bit splitting unit324splits 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 DAC2, and the most significant bit D[4] is supplied to the driver DRV4.

FIG. 10is an operation table showing operations of the optical transmitter300. The lower-bit drive unit121repeatedly 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 DAC2of the upper-bit drive unit322outputs a positive-phase output signal to the phase modulation region PM31_2and outputs a negative-phase output signal to the phase modulation region PM32_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 DAC2take 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 DRV4of the upper-bit drive unit322outputs “0” as the positive-phase output signal to the phase modulation region PM31_3, and outputs “16” as the negative-phase output signal to the phase modulation region PM32_3. On the other hand, when the most significant bit D[4] is “1”, the driver DRV4outputs “16” as the positive-phase output signal to the phase modulation region PM31_3, and outputs “0” as the negative-phase output signal to the phase modulation region PM32_3.

That is, as in the first and second embodiments, the lower-bit drive unit121(D/A converter DAC1) 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 unit121performs a fine control according to the values of the lower bits.

In the upper-bit drive unit322, the driver DRV4performs a first rough control according to the most significant bit. Further, the D/A converter DAC2performs a second rough control which is finer than the first rough control.

In other words, in the optical transmitter300, 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 transmitters100and200.

In the optical transmitter300, the number of phase modulation regions, i.e., segmented electrodes, can be reduced as compared with the optical transmitters100and200. 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 transmitters100and200, 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 DAC1of the lower-bit drive unit outputs 2n-level signals (“0” to “2n−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 “2n”.

Fourth Exemplary Embodiment

Next, an optical transmission/reception system400according to a fourth exemplary embodiment of the present invention will be described. The optical transmission/reception system400is an optical transmission/reception system using one of the above-described optical transmitters100,200, and300. An example in which the optical transmission/reception system400includes the optical transmitter100will now be described.FIG. 11is a block diagram schematically showing the configuration of the optical transmission/reception system according to the fourth exemplary embodiment.

The optical transmission/reception system400includes the optical transmitter100, an optical receiver401, an optical transmission line402, and optical amplifiers403.

The optical transmitter100outputs, 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 transmitter100and the optical receiver401are optically connected via the optical transmission line402, and the QPSK optical signal propagates therethrough. The optical amplifiers are disposed on the optical transmission line402, and amplify the QPSK optical signal propagating through the optical transmission line402. The optical receiver401demodulates the QPSK optical signal into an electric signal.

The configuration described above allows the optical transmission/reception system400to transmit the optical signal by using the optical transmitter100. The optical transmitter100can be replaced by the optical transmitter200or300, 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 transmitters100and200are described as 4-bit optical transmitters and the optical transmitter300is 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.

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