DELAY CONTROL CIRCUIT, OPTICAL TRANSMITTER, AND DELAY CONTROL METHOD

1 . A delay control circuit comprising:

a delay circuit configured to delay, by a predetermined delay, a signal input to a plurality of electrode segments provided in series along one or both of two waveguides of a Mach-Zehnder interferometer of an optical modulator;

a power monitor configured to monitor a power of a baud rate frequency component including a frequency having a value that is equal to a baud rate or an integer multiple of the baud rate, or a power of a beat frequency component of the baud rate frequency component, from output light of the optical modulator; and

a control circuit configured to control a delay amount of the delay circuit so as to maximize a monitored power of the baud rate frequency component or the beat frequency component.

2 . The delay control circuit as claimed in claim 1 , further comprising:

a baud rate frequency filter configured to extract the baud rate frequency component from an electrical output of a photodetector that detects a portion of the output light of the optical modulator,

wherein the power monitor monitors a power of the baud rate frequency component passed through the baud rate frequency filter.

3 . The delay control circuit as claimed in claim 1 , wherein:

the delay control circuit receives, as an input, a detection result of an optical frequency component that is spaced apart from a center frequency of a carrier wave included in an optical spectrum of the output light of the optical modulator by the baud rate frequency component, and

the power monitor monitors a power of an electrical signal representing the detection result.

4 . The delay control circuit as claimed in claim 1 , further comprising:

a frequency generator configured to generate a clock frequency different from the baud rate frequency component; and

a multiplier configured to multiply the baud rate frequency component by the clock frequency,

wherein the power monitor monitors a power of the beat frequency component output from the multiplier.

5 . An optical transmitter comprising:

a digital signal processor;

an optical modulator driven by signal bits output from the digital signal processor;

a delay control circuit configured to control a delay amount of the signal bits input to the optical modulator; and

a photodetector configured to detect a portion of output light of the optical modulator and output a detection result to the delay control circuit,

wherein the delay control circuit controls the delay amount of the signal bits so as to maximize a power of a baud rate frequency component including a frequency having a value equal to a baud rate or an integer multiple of the baud rate or a power of a beat frequency component of the baud rate frequency component, included in a detection result of the photodetector.

6 . The optical transmitter as claimed in claim 5 , wherein the delay control circuit includes:

a baud rate frequency filter configured to extract the baud rate frequency component from the detection result of the photodetector, and

a power monitor configured to monitor a power of the baud rate frequency component passed through the baud rate frequency filter.

7 . The optical transmitter as claimed in claim 5 , further comprising:

an optical filter configured to extract an optical frequency component spaced apart from a center frequency of a carrier wave by the baud rate frequency component from the output light of the optical modulator, wherein

the photodetector detects the optical frequency component transmitted through the optical filter, and

the delay control circuit controls the delay amount of the signal bits so as to maximize the power of the baud rate frequency component included in the detection result of the photodetector.

8 . The optical transmitter as claimed in claim 5 , wherein the delay control circuit includes:

a frequency generator configured to generate a clock frequency different from the baud rate frequency component, and

a multiplier configured to multiply the baud rate frequency component by the clock frequency, and

the delay control circuit controls the delay amount of the signal bits so as to maximize the power of the beat frequency component output from the multiplier.

9 . The optical transmitter as claimed in claim 5 , wherein:

the optical modulator includes a plurality of electrode segments provided in series along one or both of two waveguides forming a Mach-Zehnder interferometer, and

the delay control circuit controls the delay amount of each of the signal bits output from the digital signal processor.

10 . A delay control method comprising:

changing a delay amount of signals input to a plurality of electrode segments provided in series along one or both of two waveguides of a Mach-Zehnder interferometer of an optical modulator;

monitoring, from output light of the optical modulator, a power of a baud rate frequency component including a frequency having a value equal to a baud rate or an integer multiple of the baud rate, or a power of a beat frequency component of the baud rate frequency component;

setting the delay amount to a value that maximizes the power of the monitored baud rate frequency component or the monitored beat frequency component.

11 . The delay control method as claimed in claim 10 , further comprising:

detecting a portion of the output light of the optical modulator by a photodetector; and

obtaining the baud rate frequency component from an electrical output of the photodetector.

12 . The delay control method as claimed in claim 10 , further comprising:

extracting, by an optical filter, an optical frequency component that is spaced apart from a center frequency of a carrier wave included in an optical spectrum of the output light of the optical modulator by the baud rate frequency component; and

detecting the optical frequency component transmitted through the optical filter by a photodetector; and

monitoring a power of an electrical signal representing the optical frequency component from a detection result of the photodetector.

13 . The delay control method as claimed in claim 10 , further comprising:

generating a clock frequency different from the baud rate frequency component;

multiplying the baud rate frequency component by the clock frequency to extract the beat frequency component; and

monitoring a power of the beat frequency component.