Patent ID: 11888526
Assignee: FUJITSU LIMITED
Field: Optics (Instruments)
Classification: CPC H  G | IPC G  H

Claim 10:
11. A method of controlling a bias voltage of an electro-optic modulator, the electro-optic modulator being a Mach-Zehnder modulator in which a first child modulator and a second child modulator are nested to form a parent modulator, comprising:
superimposing a first dither signal with a first frequency and a second dither signal with a second frequency different from the first frequency onto the bias voltage of the electro-optic modulator in a time sharing manner;
monitoring an output light of the electro-optic modulator;
calculating a first control error based on a first component oscillating at the first frequency, and a second control error based on a second component oscillating at the second frequency from a monitoring result; and
determining a control value for controlling the bias voltage using the first control error and the second control error,
wherein the first dither signal is superimposed onto each of a first bias for the first child modulator, a second bias for the second child modulator, and a third bias for the parent modulator in a first section of a bias control cycle to detect the first component from the output light of the electro-optic modulator, and the second dither signal is superimposed onto each of the first bias, the second bias, and the third bias in a second section of the bias control cycle to detect the second component from the output light of the electro-optic modulator.