Methods and systems for tuning a resonant modulator are disclosed. One method includes receiving a carrier signal modulated by the resonant modulator with a stream of data having an approximately equal number of high and low bits, determining an average power of the modulated carrier signal, comparing the average power to a predetermined threshold, and operating a tuning device coupled to the resonant modulator based on the comparison of the average power and the predetermined threshold. One system includes an input structure, a plurality of processing elements, and a digital control element. The input structure is configured to receive, from the resonant modulator, a modulated carrier signal. The plurality of processing elements are configured to determine an average power of the modulated carrier signal. The digital control element is configured to operate a tuning device coupled to the resonant modulator based on the average power of the modulated carrier signal.

An optical transmitter includes a signal-process circuit to process a transmission signal; an optical modulator to modulate light input by the transmission signal output from the signal-process circuit, and output an optical signal; and a control circuit to output a control signal for controlling a carrier frequency of the optical signal, to the signal-process circuit, wherein the signal-process circuit comprises a phase-rotation circuit to apply phase rotation of the carrier frequency on a complex plane according to the control signal, to the transmission signal, a map-adjustment circuit to determine scale factor for a map according to an angle of the phase rotation, and a modulation-format-map circuit to map the transmission signal on the complex plane based on a modulation format and the scale factor, wherein the phase-rotation circuit is configured to rotate, on the complex plane, the phase of the carrier frequency mapped based on the scale factor.

An optical transmitter includes a signal-processing circuit configured to perform signal processing on a first transmission signal and output a second transmission signal; an optical modulator configured to modulate input light with the second transmission signal and to output an optical signal; and a control circuit configured to output a control signal for controlling a carrier frequency of the optical signal to the signal-processing circuit, wherein the signal-processing circuit includes a map-adjustment circuit configured to adjust, based on the control signal and a modulation format, a map position of the second transmission signal onto a complex plane, and a phase-rotation circuit configured to apply, on the complex plane, rotation of a phase of the carrier frequency corresponding to the control signal to the second transmission signal at the adjusted map position.

A digital instruction is generated regarding one or more electrical-to-optical conversion impairments induced at the transmitter of an optical communication system. The digital instruction may be used by the transmitter to reduce the impairments. Alternatively, or additionally, the digital instruction may be used by the receiver of the optical communication system to compensate for the impairments

A transmitter with at least one optical modulator adapted to modulate the optical signal output by a laser source to generate a modulated optical signal, wherein the optical signal output by the laser source is tapped and supplied to a monitoring circuit comprising an optical front end configured to select signal components of the tapped modulated optical signal and to convert the selected signal components of the tapped modulated optical signal into analog signals, and comprising at least one analog-to-digital converter, ADC, adapted to perform equivalent-time sampling of the analog signals to provide digital signals processed by a processing unit to monitor signal quality of the modulated optical signal.

A transmitter with at least one optical modulator adapted to modulate the optical signal output by a laser source to generate a modulated optical signal, wherein the optical signal output by the laser source is tapped and supplied to a monitoring circuit comprising an optical front end configured to select signal components of the tapped modulated optical signal and to convert the selected signal components of the tapped modulated optical signal into analog signals, and comprising at least one analog-to-digital converter, ADC, adapted to perform equivalent-time sampling of the analog signals to provide digital signals processed by a processing unit to monitor signal quality of the modulated optical signal.

A method of controlling a parameter in the generation of a coherent optical signal, the method comprising the steps of: receiving a set of signal samples relating to detection of a coherent optical signal; transforming the set of signal samples into a set of spectrum samples in the frequency domain, the set of spectrum samples being an estimation of the spectrum of the coherent optical signal; calculating at least one feedback variable based on the spectrum samples; and adjusting the parameter based on the at least one feedback variable.

A method of controlling a parameter in the generation of a coherent optical signal, the method comprising the steps of: receiving a set of signal samples relating to detection of a coherent optical signal; transforming the set of signal samples into a set of spectrum samples in the frequency domain, the set of spectrum samples being an estimation of the spectrum of the coherent optical signal; calculating at least one feedback variable based on the spectrum samples; and adjusting the parameter based on the at least one feedback variable.

An optical communication device includes an optical modulator of a Mach-Zehnder type, a low frequency superimposing circuit configured to superimpose a low frequency signal on a substrate bias voltage applied to the optical modulator, a monitor configured to monitor a modulated light output from the optical modulator, and a substrate bias controller configured to control the substrate bias voltage based upon a low frequency component contained in a monitor signal output from the monitor.

An optical communication device includes an optical modulator of a Mach-Zehnder type, a low frequency superimposing circuit configured to superimpose a low frequency signal on a substrate bias voltage applied to the optical modulator, a monitor configured to monitor a modulated light output from the optical modulator, and a substrate bias controller configured to control the substrate bias voltage based upon a low frequency component contained in a monitor signal output from the monitor.