Optical device, optical modulation method, and optical transmitter

1. An optical modulation device comprising:

a plurality of waveform shapers that waveform-shape a plurality of input data signals in synchronism with a rising or falling timing based on comparison with a reference level of an input clock signal;

a multi-level phase modulator that generates a multi-level-phase-modulated optical signal based on the plurality of data signals waveform-shaped by the plurality of waveform shapers, and outputs the generated optical signal; and

a level ratio controller that varies a relative level ratio of the reference level to an amplitude level of the clock signal input to the plurality of waveform shapers, based on the optical signal output from the multi-level phase modulator.

2. The optical modulation device according to claim 1 , wherein the level ratio controller comprises:

a varier that varies the relative level ratio of the reference level to the amplitude level of the clock signal according to a predetermined frequency;

an extractor that extracts a component of the predetermined frequency from the optical signal output from the multi-value phase modulator; and

an adjuster that adjusts a median of the relative level ratio periodically varied by the varier, based on the component of the predetermined frequency extracted by the extractor.

3. The optical modulation device according to claim 2 , wherein the varier comprises:

an oscillation circuit that generates a signal of the predetermined frequency; and

a frequency component superimposer that superimposes the signal of the predetermined frequency output from the oscillation circuit, on the clock signal input to the plurality of waveform shapers or on a reference level signal providing the plurality of waveform shapers with the reference level, and supplies the superimposition signal to the plurality of waveform shapers.

4. The optical modulation device according to claim 3 ,

wherein the adjuster adjusts the median of the relative level ratio by adjusting an DC level of a waveform of the clock signal input to the plurality of waveform shapers, so that the component of the predetermined frequency extracted by the extractor is minimum.

5. The optical modulation device according to claim 3 ,

wherein the adjuster adjusts the median of the relative reference level ratio of the reference level to the amplitude level of the clock signal input to the plurality of waveform shapers, by adjusting the reference level signal so that the component of the predetermined frequency extracted by the extractor is minimum, and supplying the adjusted reference level signal to the waveform shapers.

6. The optical modulation device according to claim 3 ,

wherein the plurality of waveform shapers are structured as two waveform shapers that waveform-shape two input data signals in synchronism with the rising or falling timing based on comparison with the reference level of the input clock signal, and

wherein a signal inverter is further provided that inverts one of the signals of the predetermined frequency supplied to the plurality of waveform shapers through the superimposition on the clock signal or on the reference level signal.

7. The optical modulation device according to claim 2 ,

wherein the extractor extracts the component of the predetermined frequency from the optical signal based on the signal of the predetermined frequency output from an oscillation circuit.

8. The optical modulation device according to claim 1 , further comprising a first RZ modulator that RZ (return to zero)-modulates the optical signal output from the multi-level phase modulator, based on the clock signal,

wherein the level ratio controller varies the relative level ratio of the reference level to the amplitude level of the clock signal input to the plurality of waveform shapers, based on a first RZ optical signal output from the first RZ modulator.

9. The optical modulation device according to claim 1 , further comprising a second RZ modulator that generates a second RZ optical signal RZ-modulated based on the clock signal, and outputs the generated signal,

wherein the multi-level phase modulator generates the multi-level-phase-modulated optical signal from the second RZ optical signal from the second RZ modulator, and outputs the generated signal.

10. The optical modulation device according to claim 8 or claim 9 ,

wherein the plurality of waveform shapers are structured as two waveform shapers that waveform-shape two input data signals in synchronism with the rising or falling timing based on comparison with the reference level of the input clock signal,

wherein the level ratio controller comprises:

a varier that varies the relative level ratio of the reference level to the amplitude level of the clock signal according to a predetermined frequency;

an extractor that extracts a component of the predetermined frequency from the optical signal output from the first RZ modulator or the multi-level phase modulator; and

an adjuster that adjusts a median of the relative level ratio periodically varied by the varier, based on the component of the predetermined frequency extracted by the extractor,

wherein the varier comprises:

an oscillation circuit that generates a signal of the predetermined frequency; and

a frequency component superimposer that superimposes the signal of the predetermined frequency output from the oscillation circuit, on the clock signal input to the two waveform shapers or on a reference level signal providing the plurality of waveform shapers with the reference level, and supplies the superimposition signal to the two waveform shapers, and

wherein a signal inverter is further provided that inverts one of the signals of the predetermined frequency oscillated by the oscillation circuit and supplied to the plurality of waveform shapers through the superimposition on the clock signal or on the reference level signal by the frequency component superimposer.

11. The optical modulation device according to claim 10 , further comprising a switcher that switches between inversion and non-inversion of the signal of the predetermined frequency at the signal inverter.

12. The optical modulation device according to claim 10 ,

wherein the extractor comprises:

a coupler that branches part of the optical signal output from the first RZ modulator or the multi-level phase modulator;

a receiver that receives the part of the optical signal branched by the coupler, and converts the received signal into an electric signal; and

a synchronous detector that extracts the component of the predetermined frequency contained in the electric signal from the receiver, by synchronous detection based on the signal of the predetermined frequency from the oscillation circuit.

13. The optical modulation device according to claim 1 ,

wherein the plurality of waveform shapers are structured as two waveform shapers that waveform-shape two input data signals in synchronism with the rising or falling timing based on comparison with the reference level of the input clock signal, and

wherein the multi-level phase modulator performs a DQPSK (differential quadrature phase shift keying) modulation based on the two data signals waveform-shaped by the two waveform shapers.

14. The optical modulation device according to claim 1 , comprising a phase shifter that applies a semi-fixed phase shift to the clock signal supplied by at least one of the plurality of waveform shapers.

15. The optical modulation device according to claim 8 , comprising a phase shifter that applies a semi-fixed phase shift to the clock signal supplied by at least one of the plurality of waveform shapers and the first RZ modulator.

16. The optical modulation device according to claim 9 , comprising a phase shifter that applies a semi-fixed phase shift to the clock signal supplied by at least one of the plurality of waveform shapers and the second RZ modulator.

17. The optical modulation device according to claim 1 , further comprising a clock signal source that generates the clock signal.

18. The optical modulation device according to claim 17 ,

wherein the clock signal source outputs a sinusoidal signal as the clock signal.

19. The optical modulation device according to claim 17 ,

wherein the clock signal source outputs a triangular signal as the clock signal.

20. The optical modulation device according to claim 17 ,

wherein the clock signal source outputs a sawtooth signal as the clock signal.

21. An optical transmitter provided with the optical modulation device according to claim 1 .

22. An optical modulation method comprising:

waveform-shaping a plurality of input data signals in synchronism with a rising or falling timing based on comparison with a reference level of an input clock signal;

generating a multi-level-phase-modulated optical signal based on the plurality of waveform-shaped data signals, and outputting the generated signal; and

varying a relative level ratio of the reference level to an amplitude level of the clock signal based on the multi-level-phase-modulated optical signal, the relative level ratio serving as a reference to determine a timing of the waveform shaping of the plurality of data signals.

23. The optical modulation method according to claim 22 ,

wherein when the relative level ratio is varied,

the relative level ratio of the reference level to the amplitude level of the clock signal in performing the waveform shaping is varied according to a predetermined frequency;

a component of the predetermined frequency is extracted from the multi-level-phase-modulated optical signal; and

a median of the periodically varied relative level ratio is adjusted based on the extracted component of the predetermined frequency.