Optical transmitter for transmitting multilevel optical signals

1. An optical transmitter that transmits a modulated optical signal in which each symbol generated by the optical transmitter carries M bits, M being an integer larger than one, the optical transmitter comprising:

a signal generation circuit configured to generate M×N binary electric signals based on transmission data, bit rates of the M×N binary electric signals being equal to each other, N being an integer larger than one, the optical transmitter multiplexes N optical signals in time-division multiplexing;

a Mach-Zehnder interferometer; and

M×N phase-shift elements provided along an optical path of the Mach-Zehnder interferometer and respectively configured to shift phases of light propagated in the optical path corresponding to the M×N binary electric signals,

the M×N phase-shift segments are comprised of N electrode groups,

each of the N electrode groups includes M or more electrodes to which corresponding M binary electric signals among the M×N binary electric signals are given,

the N electrode groups are provided in series along the optical path of the Mach-Zehnder interferometer, and

a distance between an electrode included in a first electrode group and a corresponding electrode included in a second electrode group adjacent to the first electrode group among the N electrode groups is a distance that light propagates in the optical path for a time corresponding to one time slot of the time-division multiplexing.

2. The optical transmitter according to claim 1 , wherein each of the N electrode groups is comprised of M electrodes with lengths being different from one another.

3. The optical transmitter according to claim 1 , wherein each of the N electrode groups is comprised of a number of electrodes with lengths being equal to each other, the number of electrodes being higher than M.

4. The optical transmitter according to claim 1 , wherein the signal generation circuit adjusts timings of the M×N binary electric signals so that M optical signals corresponding to the M binary electric signals given to each of the N electrode groups overlap one another in a time domain.

5. The optical transmitter according to claim 1 , wherein the signal generation circuit includes

an encoder configured to generate M×N×K sub-data sequences based on the transmission data, K being an integer larger than one,

M×N serializers,

M×N drivers, and

a clock generation circuit configured to generate a clock signal,

the M×N serializers select bits sequentially from corresponding K sub-data sequences by using the clock signal to generate M×N bit sequences, and

the M×N drivers generate the M×N binary electric signals from the M×N bit sequences.

6. An optical transmitter that transmits a modulated optical signal in which each symbol generated by the optical transmitter carries M bits, M being an integer larger than one, the optical transmitter comprising:

a signal generation circuit configured to generate M×N binary electric signals based on transmission data, bit rates of the M×N binary electric signals being equal to each other, N being an integer larger than one, wherein the optical transmitter multiplexes N optical signals in time-division multiplexing;

a Mach-Zehnder interferometer; and

M×N phase-shift elements provided along an optical path of the Mach-Zehnder interferometer and respectively configured to shift phases of light propagated in the optical path corresponding to the M×N binary electric signals,

the M×N phase-shift segments are comprised of N electrode groups,

each of the N electrode groups includes M or more electrodes to which corresponding M binary electric signals among the M×N binary electric signals are given,

the N electrode groups are provided in series along the optical path of the Mach-Zehnder interferometer, and

a distance between an electrode included in a first electrode group and a corresponding electrode included in a second electrode group adjacent to the first electrode group among the N electrode groups is a distance that light propagates in the optical path for a period M/B when a bit rate of the transmission data is B.

7. An optical transceiver including an optical receiver and an optical transmitter that transmits a modulated optical signal in which each symbol generated by the optical transmitter carries M bits, M being an integer larger than one, the optical transmitter includes

a signal generation circuit configured to generate M×N binary electric signals based on transmission data, bit rates of the M×N binary electric signals being equal to each other, N being an integer larger than one, the optical transmitter multiplexes N optical signals in time-division multiplexing,

a Mach-Zehnder interferometer, and

M×N phase-shift elements provided along an optical path of the Mach-Zehnder interferometer and respectively configured to shift phases of light propagated in the optical path corresponding to the M×N binary electric signals,

the M×N phase-shift segments are comprised of N electrode groups,

each of the N electrode groups includes M or more electrodes to which corresponding M binary electric signals among the M×N binary electric signals are given,

the N electrode groups are provided in series along the optical path of the Mach-Zehnder interferometer, and

a distance between an electrode included in a first electrode group and a corresponding electrode included in a second electrode group adjacent to the first electrode group among the N electrode groups is a distance that light propagates in the optical path for a time corresponding to one time slot of the time-division multiplexing.