Optical transmitter and bias control method
An optical transmitter includes: an optical modulator of a Mach-Zehnder type using an InP-based material; a bias controller configured to control a DC bias applied to the optical modulator; a monitor configured to monitor output light of the optical modulator to generate a monitor signal; and a corrector configured to correct a gain of the monitor signal in a direction in which wavelength dependency of insertion loss of the optical modulator is compensated according to a wavelength at a preceding stage of bias control by the bias controller.
1 . An optical transmitter comprising:
an optical modulator of a Mach-Zehnder type using an InP-based material;
a bias controller configured to control a DC bias applied to the optical modulator;
a monitor configured to monitor output light of the optical modulator to generate a monitor signal; and
a corrector configured to adjust a gain of the monitor signal such that wavelength dependency of insertion loss of the optical modulator is compensated according to a target wavelength, prior to bias control by the bias controller.
2 . The optical transmitter as claimed in claim 1 , wherein the monitor includes
a photodetector configured to detect part of the output light of the optical modulator,
a transimpedance amplifier configured to convert a photocurrent obtained by the photodetector into a voltage signal,
a band-pass filter configured to extract a predetermined frequency component included in the voltage signal that is output from the transimpedance amplifier, and
an electric amplifier configured to amplify the predetermined frequency component extracted by the band-pass filter,
wherein the corrector is configured to adjust a gain of the electric amplifier according to the target wavelength.
3 . The optical transmitter as claimed in claim 1 , wherein the monitor includes
a photodetector configured to detect part of the output light of the optical modulator,
a transimpedance amplifier configured to convert a photocurrent obtained by the photodetector into a voltage signal,
a band-pass filter configured to extract a predetermined frequency component included in the voltage signal that is output from the transimpedance amplifier, and
an electric amplifier configured to amplify the predetermined frequency component extracted by the band-pass filter,
wherein the corrector is configured to adjust a gain of the transimpedance amplifier according to the target wavelength.
4 . The optical transmitter as claimed in claim 1 , wherein
the bias controller includes a digital gain adjuster configured to digitally adjust the gain of the monitor signal, and
the corrector is configured to adjust a digital gain of the digital gain adjuster according to the target wavelength.
5 . The optical transmitter as claimed in claim 1 , wherein
the bias controller includes a wavelength channel data obtainer configured to obtain setting information on a current wavelength channel, and
the corrector is configured to determine a correction value for adjusting the gain of the monitor signal, based on the setting information obtained by the wavelength channel data obtainer.
6 . The optical transmitter as claimed in claim 1 , wherein the bias controller is configured to superimpose a dither signal on the DC bias applied to the optical modulator, and control a voltage value of the DC bias in a direction in which a phase error between a dither component included in the monitor signal having the adjusted gain and the dither signal is reduced.
7 . A bias control method comprising:
superimposing a dither signal on a DC bias of an optical modulator of a Mach-Zehnder type using an InP-based material; and
monitoring output light of the optical modulator to generate a monitor signal including a dither component that fluctuates at a same frequency as the dither signal;
adjusting a gain of the monitor signal according to a target wavelength so as to compensate for wavelength dependency of insertion loss of the optical modulator; and
executing bias control of the optical modulator using the adjusted monitor signal.
8 . The bias control method as claimed in claim 7 , the method further comprising detecting part of the output light of the optical modulator by a photodetector, and adjusting a gain of a transimpedance amplifier configured to convert a photocurrent output from the photodetector into a voltage signal according to the target wavelength.
9 . The bias control method as claimed in claim 7 , the method further comprising extracting a dither component fluctuating at a same frequency as the dither signal from the output light of the optical modulator, and adjusting a gain of an electric amplifier amplifying the dither component according to the target wavelength.
10 . The bias control method as claimed in claim 7 , the method further comprising converting the monitor signal into a digital signal, and adjusting a digital gain of the digital signal according to the target wavelength.