Optical transmitter that includes optical modulator

An optical transmitter includes: an optical modulator, a phase adjustment circuit, first and second synchronization circuits, and first and second drive circuits. The optical modulator includes a first modulation area and a second modulation area that is provided at output side of the first modulation area. The phase adjustment circuit adjusts a phase of a first clock signal so as to generate a second clock signal. The first and second synchronization circuits respectively output first and second electric signals in synchronization with the first and second clock signals. The first and second drive circuits respectively drive the first and second modulation areas with the first and second electric signals.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2017-087884, filed on Apr. 27, 2017, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to an optical transmitter that includes an optical modulator driven by a plurality of electric signals.

BACKGROUND

An optical transmitter including a Mach-Zehnder modulator driven by an electric signal is known as an example of an optical transmitter that provides a high-speed data communication (for example, Japanese Laid-open Patent Publication No. 2014-138361 and Document 1). Continuous wave light is input to the Mach-Zehnder modulator. Then, a modulated optical signal is generated by driving the Mach-Zehnder modulator with an electric signal indicating transmission data.

FIG. 1illustrates an example of an optical transmitter including a Mach-Zehnder modulator. In this example, the optical transmitter includes a Mach-Zehnder modulator1, a driver2a, and a driver2b. A modulation area of the Mach-Zehnder modulator1is divided into a plurality of modulation areas. In the example illustrated inFIG. 1, the Mach-Zehnder modulator1includes a modulation area1aand a modulation area1b. Non-modulated continuous wave light is input to the Mach-Zehnder modulator1. The driver2agenerates a drive signal (a) from data (a), and the driver2bgenerates a drive signal (b) from data (b). Then, the drive signal (a) and the drive signal (b) are respectively applied to the modulation area1aand the modulation area1b. This configuration provides a pulse-amplitude modulation (PAM) that transmits the data (a) and the data (b). In the configuration illustrated inFIG. 1, 4-level pulse-amplitude modulation (PAM4) is provided if the length of the modulation area1bis twice the length of the modulation area1a(for example, Document 2).

In 4-level pulse-amplitude modulation, for example, the following optical amplitudes A are obtained for a combination of data (a) and data (b).“a=0, b=0”: “A=0”“a=1, b=0”: “A=1”“a=0, b=1”: “A=2”“a=1, b=1”: “A=3”

It is assumed that, in the optical transmitter illustrated inFIG. 1, the drive signals (a) and (b) are given to the Mach-Zehnder modulator1at the same timing. However, light input to the Mach-Zehnder modulator1passes through the modulation area1aand then passes through the modulation area1b. Thus, the timing at which the input light is modulated by the drive signal (b) in the modulation area1bis shifted, by a light propagation delay time τ, with respect to the timing at which the input light is modulated by the drive signal (a) in the modulation area1a. τ depends on the length of the modulation area1a. The strength to modulate an optical signal depends on the strength of a drive signal. Thus, the waveform of a modulated optical signal output from the Mach-Zehnder modulator1will be distorted if the timings at which input light is modulated are different.

This problem may be solved if a drive signal (b) output from the driver2bis delayed by a time τ with respect to a drive signal (a). Delaying an electric signal given to a Mach-Zehnder modulator is disclosed in, for example, Document 3. Further, an optical transmitter that can operate normally even if a data-transmission speed varies is disclosed in, for example, Japanese Laid-open Patent Publication No. 2003-218790.

REFERENCES

However, in conventional technologies (for example, a technology disclosed in Document 3), an electric signal output from a driver is delayed directly. Specifically, the timing of applying, to each modulation area, an electric signal output from a driver is adjusted by changing the number of amplifiers through which the electric signal passes. Alternatively, the timing of applying, to each modulation area, an electric signal output from a driver is adjusted by changing the length of a transmission line through which the electric signal passes. Thus, the waveform of an electric signal (the drive signals (a) and (b) inFIG. 1) applied to a Mach-Zehnder modulator is deteriorated due to bandwidth of an amplifier or a transmission line. In this case, the waveform of a modulated optical signal output from the Mach-Zehnder modulator1may also be deteriorated.

SUMMARY

According to an aspect of the present invention, an optical transmitter includes: an optical modulator equipped with a first arm and a second arm, the first arm including a first modulation area and a second modulation area that is provided at output side of the first modulation area, and the second arm including a third modulation area and a fourth modulation area respectively corresponding to the first modulation area and the second modulation area; a phase adjustment circuit configured to adjust a phase of a first clock signal so as to generate a second clock signal; a first synchronization circuit configured to output a first electric signal in synchronization with the first clock signal; a second synchronization circuit configured to output a second electric signal in synchronization with the second clock signal; a first drive circuit configured to drive the first modulation area and the third modulation area with the first electric signal output from the first synchronization circuit; and a second drive circuit configured to drive the second modulation area and the fourth modulation area with the second electric signal output from the second synchronization circuit.

DESCRIPTION OF EMBODIMENTS

First Embodiment

FIG. 2illustrates an example of an optical transmitter according to a first embodiment of the present invention. As illustrated inFIG. 2, an optical transmitter1000according to the first embodiment includes an optical modulator10, a phase adjustment circuit20, a multiplexer circuit30, and a drive circuit40. The optical transmitter1000may include other circuit elements not illustrated inFIG. 2.

For example, non-modulated continuous wave light is input to the optical transmitter1000. Continuous wave light is generated by a light source (not illustrated). This light source is implemented by, for example, a laser source that generates continuous wave light of a specified wavelength. Further, a clock signal CLK is input to the optical transmitter1000. A clock signal is generated by a clock signal generation circuit (not illustrated). The frequency of a clock signal is determined, for example, according to a bit rate of data transmitted by the optical transmitter1000. Further, an electric signal S1that indicates data1and an electric signal S2that indicates data2are input to the optical transmitter1000. In this example, the electric signal S1and the electric signal S2are respectively parallel signals that transmit a plurality of bit streams. The optical transmitter1000generates a modulated optical signal based on the electric signal S1and the electric signal S2.

In this example, the optical modulator10is implemented by the Mach-Zehnder modulator illustrated inFIG. 3. Specifically, the optical modulator10includes an input optical waveguide11, a first arm optical waveguide12, a second arm optical waveguide13, and an output optical waveguide14. The input optical waveguide11guides input continuous wave light to the first arm optical waveguide12and the second arm optical waveguide13. Light propagated through the first arm optical waveguide12and light propagated through the second arm optical waveguide13are combined and guided to the output optical waveguide14. Here, the input continuous wave light is modulated in the first arm optical waveguide12and the second arm optical waveguide13. As a result, a modulated optical signal is generated.

A drive signal DR1and a drive signal DR2are applied to the optical modulator10. In this example, the drive signal DR1and the drive signal DR2are respectively differential signals, which will be described later.

Signal electrodes15and16are formed near the first arm optical waveguide12. Here, the signal electrode16is formed at the output side of the signal electrode15. The drive signal DR1is applied to the signal electrode15, and the drive signal DR2is applied to the signal electrode16. Thus, light propagated through the first arm optical waveguide12is modulated by the drive signal DR1applied to the signal electrode15, and is modulated by the drive signal DR2applied to the signal electrode16. Likewise, signal electrodes17and18are formed near the second arm optical waveguide13. Here, the signal electrode18is formed at the output side of the signal electrode17. The drive signal DR1is applied to the signal electrode17, and the drive signal DR2is applied to the signal electrode18. Thus, light propagated through the second arm optical waveguide13is modulated by the drive signal DR1applied to the signal electrode17, and is modulated by the drive signal DR2applied to the signal electrode18. InFIG. 3, a bias electrode that adjusts an operating point of the optical modulator10is omitted.

A drive signal DR1is applied to the signal electrodes15and17. Here, the drive signal DR1is a differential signal and is formed by a pair of equal and opposite signals. One of the pair of the signals in the drive signal DR1is applied to the signal electrode15and the other one of the pair of the signals in the drive signal DR1is applied to the signal electrode17. Thus, the first arm optical waveguide12, the second arm optical waveguide13, and the signal electrodes15and17configure a modulation unit101. Note that the length of the signal electrode15and the length of the signal electrode17are the same as each other.

Likewise, a drive signal DR2is applied to the signal electrodes16and18. Here, the drive signal DR2is also a differential signal and is formed by a pair of equal and opposite signals. One of the pair of the signals in the drive signal DR2is applied to the signal electrode16and the other one of the pair of the signals in the drive signal DR2is applied to the signal electrode18. Thus, the first arm optical waveguide12, the second arm optical waveguide13, and the signal electrodes16and18configure a modulation unit102. Note that the length of the signal electrode16and the length of the signal electrode18are the same as each other. In addition, in this example, the lengths of the signal electrodes16and18are respectively twice the lengths of the signal electrodes15and17in order to provide PAM4.

As described above, the optical modulator10illustrated inFIG. 3includes the modulation unit101and the modulation unit102. With respect to the first arm, a modulation area for the modulation unit101is formed near the signal electrode15, and a modulation area for the modulation unit102is formed near the signal electrode16. With respect to the second arm, a modulation area for the modulation unit101is formed near the signal electrode17, and a modulation area for the modulation unit102is formed near the signal electrode18.

A clock signal CLK is input to the phase adjustment circuit20. The phase adjustment circuit20generates a clock signal CLK1and a clock signal CLK2based on the clock signal CLK. The clock signal CLK2is delayed by a time Td with respect to the clock signal CLK1. In other words, the phase of the clock signal CLK2is delayed, by a phase φ, with respect to the phase of the clock signal CLK1. The phase φ corresponds to the time Td. The clock signal CLK2may be generated by delaying the clock signal CLK1by the time Td.

The time Td corresponds to a difference between a time needed to propagate light from an input end of the optical modulator10to the modulation area101, and a time needed to propagate light from the input end of the optical modulator10to the modulation area102. Alternatively, the time Td corresponds to a propagation delay time that occurs by the arrival of light input to the optical modulator10at the modulation unit102. In other words, the time Td corresponds to a time needed for input light to pass through the modulation unit101. An example of the phase adjustment circuit20will be described later.

The multiplexer circuit30includes a multiplexer31-1and a multiplexer31-2. The multiplexer31-1multiplexes an electric signal S1so as to generate a data signal D1. Here, the electric signal S1is configured by two differential signals that are transmitted in parallel. In other words, the electric signal S1is a 4-lane parallel signal. Then, the multiplexer31-1time-division multiplexes the two differential signals included in the electric signal S1and outputs the data signal D1. Likewise, the multiplexer31-2multiplexes an electric signal S2so as to generate a data signal D2. Here, the electric signal S2is also configured by two differential signals that are transmitted in parallel. In other words, the electric signal S2is also a 4-lane parallel signal. Then, the multiplexer31-2time-division multiplexes the two differential signals included in the electric signal S2and outputs the data signal D2. Each of the data signals D1and D2is a differential signal.

FIGS. 4A and 4Billustrate examples of a configuration and an operation of a multiplexer. A multiplexer31illustrated inFIG. 4Acorresponds to the multiplexers31-1and31-2illustrated inFIG. 2. The multiplexers31-1and31-2have substantially the same configuration and perform substantially the same operation.

The multiplexer31includes flip-flop circuits32and33, and a selector34. Electric signals (the electric signal S1or the electric signal S2illustrated inFIG. 2) are input to the multiplexer31. The input electric signals are configured by a differential signal X and a differential signal Y.

The differential signal X is input to a data terminal of the flip-flop circuit32. The flip-flop circuit32holds a signal given to its data terminal using a rising edge of a clock signal. On the other hand, the differential signal Y is input to a data terminal of the flip-flop circuit33. The flip-flop circuit33holds a signal given to its data terminal using a falling edge of the clock signal. The selector34selects an output signal of the flip-flop circuit32when the state of a clock signal is H level, and selects an output signal of the flip-flop circuit33when the state of the clock signal is L level.

FIG. 4Bis a timing chart of the multiplexer31illustrated inFIG. 4A. In this example, a differential signal Xb and a differential signal Yb are alternately output from the flip-flop circuits32and33in synchronization with a clock signal. Specifically, the differential signal Xb is output in synchronization with a rising edge of a clock signal, and the differential signal Yb is output in synchronization with a falling edge of the clock signal. In other words, the differential signal Xb and the differential signal Yb are time-division multiplexed. As a result, symbols K, K+1, K+2, K+3, . . . of a data signal output from the multiplexer31respectively transmit X1in the differential signal Xb, Y1in the differential signal Yb, X2in the differential signal Xb, Y2in the differential signal Yb, . . . . As described above, the multiplexer31operates as a synchronization circuit that controls output timings of differential signals.

The drive circuit40includes a driver41-1and a driver41-2. The driver41-1generates a drive signal DR1based on the data signal D1output from the multiplexer31-1. Likewise, the driver41-2generates a drive signal DR2based on the data signal D2output from the multiplexer31-2. Each of the drive signals DR1and DR2is a differential signal.

FIG. 5illustrates a delay of a drive signal. In the optical transmitter1000, a clock signal CLK2is delayed by a time Td with respect to a clock signal CLK1, as illustrated inFIG. 5. The multiplexer31-1multiplexes the electric signal S1in synchronization with the clock signal CLK1, and the multiplexer31-2multiplexes the electric signal S2in synchronization with the clock signal CLK2. The driver41-1generates a drive signal DR1from an output signal of the multiplexer31-1, and the driver41-2generates a drive signal DR2from an output signal of the multiplexer31-2. Therefore, the drive signal DR2is delayed by the time Td with respect to the drive signal DR1.

The drive signal DR1is given to the modulation unit101of the optical modulator10. Specifically, as illustrated inFIG. 3, one of a pair of signals in the differential drive signal DR1(for example, a non-inverted signal) is applied to the electrode15that is formed near the first arm optical waveguide12, and the other one of the pair of signals in the differential drive signal DR1(for example, an inverted signal) is applied to the electrode17that is formed near the second arm optical waveguide13. In addition, the drive signal DR2is given to the modulation unit102of the optical modulator10. Specifically, as illustrated inFIG. 3, one of a pair of signals in the differential drive signal DR2(for example, a non-inverted signal) is applied to the electrode16that is formed near the first arm optical waveguide12, and the other one of the pair of signals in the differential drive signal DR2(for example, an inverted signal) is applied to the electrode18that is formed near the second arm optical waveguide13.

In the optical transmitter1000having the configuration described above, a drive signal DR1is generated based on data1, and input light is modulated by this drive signal DR1. In addition, a drive signal DR2is generated based on data2, and the input light is further modulated by this drive signal DR2. Thus, a modulated optical signal generated by the optical modulator10can transmit the data1and the data2. Here, a symbol of the data1and a symbol of the data2are multiplexed in each symbol of this modulated optical signal. In the example illustrated inFIG. 5, a symbol K of the drive signal DR1and a symbol K of the drive signal DR2are multiplexed so that a symbol K of the modulated optical signal is generated.

Here, the timing at which input light of the optical modulator10arrives at the modulation unit102is delayed by a time Td with respect to the timing at which the input light arrives at the modulation unit101. However, in the optical transmitter1000illustrated inFIG. 2, the clock signal CLK2is delayed by the time Td with respect to the clock signal CLK1. In this case, the drive signal DR2generated in synchronization with the clock signal CLK2is delayed by the time Td with respect to the drive signal DR1in synchronization with the clock signal CLK1. Thus, a modulation by the drive signal DR1and a modulation by the drive signal DR2can be accurately superimposed on each other in the optical modulator10. As a result, the characteristics of a modulated optical signal output from the optical modulator10are improved. For example, the opening of an eye pattern of a modulated optical signal becomes wider.

FIGS. 6A and 6Billustrate an example of an operation of the optical modulator10. In this example, it is assumed that a light component L passing through the modulation unit101at a time T1passes through the modulation unit102at a time T1+Td. It is also assumed that a symbol of a data signal D1output from the multiplexer31-1and a symbol of a data signal D2output from the multiplexer31-2are multiplexed by PAM4 so that a symbol of a modulated optical signal is generated.

As illustrated inFIG. 6A, a drive signal DR1representing a symbol K of the data signal D1is applied to the modulation unit101at the time T1. By doing this, the light component L is modulated in the modulation unit101by the drive signal DR1representing a symbol K of the data signal D1.

As illustrated inFIG. 6B, the light component L arrives at the modulation unit102at the time T1+Td. Here, a drive signal DR2is delayed by a time Td with respect to the drive signal DR1. Thus, the drive signal DR2representing a symbol K of the data signal D2is applied to the modulation unit102at the time T1+Td. By doing this, the light component L is modulated in the modulation unit102by the drive signal DR2representing a symbol K of the data signal D2. In other words, the light component L is modulated in the modulation unit101according to a symbol K of the data signal D1, and is then modulated in the modulation unit102according to a symbol K of the data signal D2. Here, the drive signal DR2is delayed by the time Td with respect to the drive signal DR1, so the symbol K of the data signal D1and the symbol K of the data signal D2are accurately superimposed on each other. As a result, the characteristics of a modulated optical signal output from the optical modulator10are improved.

It is preferable that, in the configuration illustrated inFIG. 2, the length of a signal line between the phase adjustment circuit20and the multiplexer31-1, and the length of a signal line between the phase adjustment circuit20and the multiplexer31-2be the same as each other. Further, it is preferable that the length of a signal line between the multiplexer31-1and the driver41-1, and the length of a signal line between the multiplexer31-2and the driver41-2be the same as each other. Furthermore, it is preferable that the length of a signal line between the driver41-1and the modulation unit101, and the length of a signal line between the driver41-2and the modulation unit102be the same as each other.

In the example illustrated inFIG. 2, the optical modulator10is driven by differential signals, but the embodiments of the present invention are not limited to this configuration. In other words, the optical modulator10may have a configuration in which a drive signal is applied to only one of the paired arms.

In the example illustrated inFIG. 2, a signal synchronized with a clock signal is generated using a multiplexer, but the embodiments of the present invention are not limited to this configuration. In other words, an electric signal synchronized with a clock signal may be generated without multiplexing the electric signal. Alternatively, an electric signal synchronized with a clock signal may be generated at the output side of a multiplexer.

Example of Phase Adjustment Circuit20

In the example illustrated inFIG. 7, a clock signal is delayed using a transmission line that propagates an electric signal. A transmission line21is formed by, for example, a conductor pattern that is formed on a substrate. The length of the transmission line21is determined such that a propagation time of light in the transmission line21is Td. This configuration does not include an active device, thus it is possible to reduce power consumption of the phase adjustment circuit20.

In the example illustrated inFIG. 8, a clock signal is delayed using an inverter device that inverts a logic of an electric signal. The delay time in an inverter device22can be designed to be a desired length. Thus, the number of invert devices22series-connected to one another is determined according to a delay time Td. For example, the phase adjustment circuit20is designed such that the product of a delay time in the inverter device22and the number of series-connected inverter devices22is Td. This configuration makes it possible to reduce the circuit area of the phase adjustment circuit20. Note that in the configuration illustrated inFIG. 8, a buffer device may be implemented instead of an inverter device. The buffer device does not invert a logic of an electric signal.

In the example illustrated inFIG. 9, a clock signal is delayed using a tri-state inverter. The delay time in a tri-state inverter23is controlled by delay amount control signals CX and CY. Specifically, the delay time in each tri-state inverter23is controlled such that the delay time of a clock signal CLK2with respect to the clock signal CLK1is Td. This configuration makes it possible to control the delay time of a clock signal CLK2with respect to a clock signal CLK1. Note that in the configuration illustrated inFIG. 9, a tri-state buffer may be implemented instead of a tri-state inverter.

In the example illustrated inFIGS. 10A and 10B, a clock signal is delayed using a transistor pair differential amplifier. In this case, as illustrated inFIG. 10A, transistor pair differential amplifiers24are series-connected to one another. As illustrated inFIG. 10B, the delay time in each of the transistor pair differential amplifiers24is adjusted by controlling a bias current and/or a tail current. Specifically, a bias current and/or a tail current of each of the transistor pair differential amplifiers24are controlled such that the delay time of a clock signal CLK2with respect to a clock signal CLK1is Td. The bias current and/or the tail current of the transistor pair differential amplifier24are controlled by a delay amount control signal. This configuration makes it possible to control the delay time of a clock signal CLK2with respect to a clock signal CLK1.

In the example illustrated inFIG. 11, a clock signal is delayed using a phase interpolator. For example, the phase adjustment circuit20includes phase interpolators25-1and25-2. A clock signal CLK_X and a clock signal CLK_Y whose phases are different from each other are given to the phase adjustment circuit20. Based on the clock signal CLK_X and the clock signal CLK_Y, the phase interpolator25-1can generate a clock signal CLK1having a phase specified by a delay amount control signal C1. Likewise, based on the clock signal CLK_X and the clock signal CLK_Y, the phase interpolator25-2can generate a clock signal CLK2having a phase specified by a delay amount control signal C2. Thus, in the configuration illustrated inFIG. 11, the delay amount control signals C1and C2are generated and given to the phase adjustment circuit20, such that the delay time of a clock CLK2with respect to a clock signal CLK1is Td. This configuration makes it possible to control the delay time of a clock signal CLK2with respect to a clock signal CLK1.

As described above, in the phase adjustment circuit20illustrated inFIGS. 9 to 11, the delay time of a clock signal CLK2with respect to a clock signal CLK1can be controlled according to a delay amount control signal. Thus, the phase adjustment circuit20may be configured to be controlled according to a modulated optical signal output from the optical modulator10. For example, the state of the tri-state inverter23illustrated inFIG. 9, the state of the transistor pair differential amplifier24illustrated inFIGS. 10A and 10B, or the state of the phase interpolator25-1,25-2is adjusted by a feedback control such that the waveform of a modulated optical signal output from the optical modulator10is optimized (for example, such that the opening of an eye pattern of a modulated optical signal becomes wider). This feedback control is performed, for example, before the optical transmitter1000is shipped. Alternatively, this feedback control may be performed when the optical transmitter1000is in use.

Second Embodiment

FIG. 12illustrates an example of an optical transmitter according to a second embodiment of the present invention. In an optical transmitter2000according to the second embodiment, the optical modulator10includes three modulation units (101-103). The modulation unit102is provided at the output side of the modulation unit101, and the modulation unit103is provided at the output side of the modulation unit102. The lengths of the modulation units101,102, and103are the same as one another.

The timing at which light input to the optical modulator10arrives at the modulation unit102is delayed by a time Td1with respect to the timing at which the input light arrives at the modulation unit101. Further, the timing at which light input to the optical modulator10arrives at the modulation unit103is delayed by a time Td2with respect to the timing at which the input light arrives at the modulation unit2. In other words, the timing at which light input to the optical modulator10arrives at the modulation unit103is delayed by a time Td1+Td2with respect to the timing at which the input light arrives at the modulation unit101.

The phase adjustment circuit20generates clock signals CLK1to CLK3based on a clock signal CLK. The clock signal CLK2is generated so as to be delayed by the time Td1with respect to the clock signal CLK1. The clock signal CLK3is generated so as to be delayed by the time Td2with respect to the clock signal CLK2.

The multiplexers31-1to31-3respectively multiplex electric signals S1to S3in synchronization with the clock signals CLK1to CLK3. In other words, the multiplexer31-1outputs a data signal D1in synchronization with the clock signal CLK1, the multiplexer31-2outputs a data signal D2in synchronization with the clock signal CLK2, and the multiplexer31-3outputs a data signal D3in synchronization with the clock signal CLK3. The drivers41-1to41-3respectively generate drive signals DR1to DR3from output signals of the multiplexers31-1to31-3(that is, the data signals D1to D3). Then, the drive signals DR1to DR3are respectively applied to the modulation units101to103.

This configuration makes it possible to accurately superimpose a modulation by the drive signal DR1, a modulation by the drive signal DR2, and a modulation by the drive signal DR3on one another. As a result, the characteristics of a modulated optical signal output from the optical modulator10are improved.

Further, in the configuration illustrated inFIG. 12, PAM4 can be realized by making the lengths of the modulation units101to103be the same as one another, and configuring the drivers41-1to41-3be the same as one another. Thus, this configuration makes it possible to design and adjust a circuit more easily if PAM4 is applied, compared to the configuration illustrated inFIG. 2.

Third Embodiment

FIG. 13illustrates an example of an optical transmitter according to a third embodiment of the present invention. In an optical transmitter3000according to the third embodiment, the optical modulator10includes n modulation units (101to10n). n is an arbitrary integer greater than or equal to two. The modulation units101to10nare sequentially provided from an input end to an output end of the optical modulator10. In this example, the lengths of the modulations units101to10nare the same as one another.

The phase adjustment circuit20generates clock signals CLK1, and CLK2to CLKn based on a clock signal CLK. The clock signals CLK2to CLKn are generated so as to be delayed by respective specified times with respect to the clock signal CLK1. In other words, a plurality of clock signals CLK1to CLKn whose phases are different from one another are generated according to the arrangement of a plurality of modulation areas101to10n. Specifically, when the difference between a time needed to propagate light from an input end of the optical modulator10to the modulation unit101and a time needed to propagate light from the input end of the optical modulator10to the modulation unit10iis Ti (i=2, 3, . . . , n), the clock signal CLKi is delayed by the time Ti with respect to the clock signal CLK1.

The multiplexers31-1to31-nrespectively multiplex electric signals S1to Sn in synchronization with the clock signals CLK1to CLKn. In other words, data signals D1to Dn are output in synchronization with the respective clock signals CLK1to CLKn. The drivers41-1to41-nrespectively generate drive signals DR1to DRn from output signals of the multiplexers31-1to31-n. Then, the drive signals DR1to DRn are respectively applied to the modulation units101to10n.

This configuration makes it possible to accurately superimpose modulations by the drive signals DR1to DRn on one another. As a result, the characteristics of a modulated optical signal output from the optical modulator10are improved. Further, the third embodiment makes it possible to apply a pulse-amplitude modulation of a desired number of bits per symbol (for example, PAM8 and PAM16) according to the value of n.

Fourth Embodiment

FIG. 14illustrates an example of an optical transmitter according to a fourth embodiment of the present invention. As illustrated inFIG. 14, an optical transmitter4000according to the fourth embodiment includes an optical modulator60, the phase adjustment circuit20, the multiplexers31-1to31-3, and the drivers41-1to41-3. The phase adjustment circuit20, the multiplexers31-1to31-3, and the drivers41-1to41-3in the fourth embodiment illustrated inFIG. 14are substantially the same as those in the second embodiment illustrated inFIG. 12.

FIG. 15illustrates an example of the optical modulator60used in the fourth embodiment. In the optical modulator60, a signal electrode61is provided near the first arm optical waveguide12, and a signal electrode62is provided near the second arm optical waveguide13. Then, drive signals DR1to DR3are applied to the optical modulator60. The drive signals DR1to DR3are generated similarly to the example illustrated inFIG. 12. Thus, the drive signal DR2is delayed by a specified time with respect to the drive signal DR1, and the drive signal DR3is delayed by a specified time with respect to the drive signal DR2.

The drive signals DR1to DR3are respectively applied to points that are physically different from one another. Specifically, the drive signal DR2is applied to a point at the output side of the applied point of the drive signal DR1, and the drive signal DR3is applied to a point at the output side of the applied point of the drive signal DR2, as illustrated inFIG. 15. The delay time of a clock signal CLK2with respect to a clock signal CLK1corresponds to a distance between the applied point of the drive signal DR1and the applied point of the drive signal DR2. The delay time of a clock signal CLK3with respect to the clock signal CLK2corresponds to a distance between the applied point of the drive signal DR3and the applied point of the drive signal DR3.

The drive signals DR1to DR3applied to the optical modulator10are attenuated when they are propagated through the signal electrodes61and62. Thus, as illustrated inFIG. 15, the interference between the drive signals DR1to DR3is small in the signal electrodes61and62. Thus, the modulation units101to103can modulate propagated light substantially independently of one another. In other words, an area (an area1) that is located around the applied point of the drive signal DR1works as the modulation unit101illustrated inFIG. 12. An area (an area2) that is located around the applied point of the drive signal DR2works as the modulation unit102illustrated inFIG. 12. An area (an area3) that is located around the applied point of the drive signal DR3works as the modulation unit103illustrated inFIG. 12.

In the example illustrated inFIGS. 14 and 15, a set of three drive signals DR1to DR3are applied to an optical modulator, but the fourth embodiment is not limited to this configuration. In other words, any number of drive signals may be given to the optical modulator in a configuration in which an electrode provided near each arm of the optical modulator is not divided into a plurality of electrodes.

The phase adjustment circuit20illustrated inFIGS. 7 to 11is also applicable to the second, third, and fourth embodiments. In the configurations illustrated inFIGS. 7 to 10B, each clock signal is generated from another clock signal. For example, a clock signal CLK1is generated from a clock signal CLK, a clock signal CLK2is generated from the clock signal CLK1, and a clock signal CLK3is generated from the clock signal CLK2. In the configuration illustrated inFIG. 11, clock signals CLK1to CLKn are generated from two input clock signals CLK_X and CLK_Y.