OPTICAL TRANSMITTER AND BIAS CONTROL METHOD FOR OPTICAL MODULATOR

• US 20160156416A1
• Filed: 11/06/2015
• Published: 06/02/2016
• Est. Priority Date: 11/27/2014
• Status: Active Grant

• Current Assignee: Fujitsu Optical Components Limited (Fujitsu Limited)
• Original Assignee: Fujitsu Optical Components Limited (Fujitsu Limited)
• Inventors: SHIRAKAWA, Iori
• Granted Patent: US 9,853,736 B2
• Time in Patent Office: Days
• CPC Class Codes: H04B 10/0799 Monitoring line transmitter or line receiver equipment
• CPC Class Codes: H04B 10/40 Transceivers
• CPC Class Codes: H04B 10/5053 using a parallel, i.e. shunt, combination of modulators
• CPC Class Codes: H04B 10/50572 to control the modulating signal amplitude including amplitude distortion
• CPC Class Codes: H04B 10/50575 to control the modulator DC bias
• CPC Class Codes: H04B 10/5161 Combination of different modulation schemes
• CPC Class Codes: H04B 10/564 Power control

Abstract

An optical transmitter has an optical modulator having Mach-Zehnder interferometers, modulator drivers configured to drive the optical modulator by a drive signal, a low frequency generator configured to generated a low frequency signal that changes a ratio of a driving amplitude with respect to a half-wave voltage of the optical modulator, a photodetector configured to detect a portion of output light of the optical modulator, a detector configured to detect a low frequency component contained in a detected signal from the photodetector using the low frequency signal, and a bias voltage controller configured to control a bias voltage for the optical modulator such that the detected low frequency component becomes the maximum and in-phase with the superimposed low frequency signal.

Claims

1. An optical transmitter comprising:

an optical modulator having Mach-Zehnder interferometers, modulator drivers configured to drive the optical modulator by drive signals,

a low frequency generator configured to generated a low frequency signal that changes a ratio of a driving amplitude with respect to a half-wave voltage of the optical modulator;

a photodetector configured to detect a portion of output light of the optical modulator;

a detector configured to detect a low frequency component contained in a detected signal from the photodetector using the low frequency signal; and

a bias voltage controller configured to control a bias voltage for the optical modulator such that the detected low frequency component becomes the maximum and in-phase with the superimposed low frequency signal.

2. The optical transmitter according to claim 1,

wherein the low frequency signal is superimposed on the drive signals generated by the modulator drives, and

wherein the optical modulator is driven by a driving waveform in which peak edges of the drive signal change in an amplitude direction symmetrically with respect to a center of the amplitude of the drive signal, at a frequency of the low frequency signal.

3. The optical transmitter according to claim 1,

wherein the Mach-Zehnder interferometers are semiconductor optical modulators of a Mach-Zehnder type, and

wherein the low frequency signal is superimposed on a substrate bias voltage applied to the semiconductor optical modulators, thereby changing the ratio of the driving amplitude with respect to the half-wave voltage symmetrically with respect to a center of the driving amplitude.

4. The optical transmitter according to claim 1, further comprising:

an amplitude detector configured to detect an amplitude level of output signals from the modulator drivers; and

an amplitude controller configured to control an amplitude of a drive signal generated by the modular drivers so as to be constant based upon a detection result of the peak detector.

5. The optical transmitter according to claim 2, further comprising:

an amplitude detector configured to detect an amplitude level of output signals from the modulator drivers; and

an amplitude controller configured to control the amplitude of the drive signal generated by the modular drivers so as to be constant based upon a detection result of the peak detector.

6. The optical transmitter according to claim 3, further comprising:

an amplitude detector configured to detect an amplitude level of output signals from the modulator drivers; and

an amplitude controller configured to control an amplitude of a drive signal generated by the modular drivers so as to be constant based upon a detection result of the peak detector.

7. The optical transmitter according to claim 1, further comprising:

an amplitude controller configured to control a magnitude of the low frequency signal to be superimposed on the drive signal based upon a detection result of the amplitude detector.

8. The optical transmitter according to claim 2, further comprising:

an amplitude controller configured to control a magnitude of the low frequency signal to be superimposed on the drive signal based upon a detection result of the amplitude detector.

9. The optical transmitter according to claim 1, further comprising:

a signal processor configured to generate an electric signal to be supplied to the modulator drivers,

wherein the electric signal is a narrowband signal having undergone Nyquist filtering or a multi-level signal produced by digital signal processing.

10. An optical transceiver comprising:

the optical transmitter according to claim 1; and

a frontend receiver configured to externally receive an optical signal and process the optical signal.

11. A bias control method for an optical modulator, comprising:

when driving an optical modulator having Mach-Zehnder interferometers, supplying a low frequency signal for changing a ratio of a driving amplitude with respect to a half-wave voltage of the optical modulator;

detecting a low frequency component contained in an output of the optical modulator using the low frequency signal; and

controlling a bias voltage for the optical modulator such that the low frequency component becomes the maximum and in-phase with the low frequency signal.