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 frontend configured to select signal components of the tapped modulated optical signal, via optical coherent detection, 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.

Disclosed herein is a dual parallel Mach-Zehnder-modulator (DPMZM) device comprising a DPMZM 10 having first and second inner MZMs arranged parallel to each other. The first inner MZM generates an in-phase component E_I of an optical signal in response to a first driving voltage V_I, and the second inner MZM generates a quadrature component E_Q of said optical signal in response to a second driving voltage V_Q. Further disclosed is a calculation unit 52 configured for receiving an in-phase component y_I and a quadrature component y_Q of a desired base-band signal, and for calculating pre-distorted first and second driving voltages V_I, V_Q. The calculation of the pre-distorted first and second driving voltages V_I, V_Q is based on a model of said DPMZM 10 accounting for I-Q cross-talk, and using an algorithm that determines said first and second driving voltages V_I, V_Q each as a function of both of said in-phase and quadrature components y_I, y_Q of said base-band signal.

An all-optical modulator(1) includes a directional coupling unit(11) that multiplexes, on a carrier light beam, a signal light beam of a modulation signal that a carrier signal at a characteristic frequency is modulated with an information signal. The all-optical modulator includes a nonlinear medium(12) that cross-phase-modulates the carrier light beam with the multiplexed signal light beam, and frequency-multiplexes the information signal in the signal light beam on the carrier light beam to generate an optical frequency division multiplexed signal. The all-optical modulator includes a monitoring control unit(15) that controls a polarization control unit(19) that controls a polarization state of the signal light beam in a direction in which an intensity of a modulation component takes a maximum value, based on the intensity of the modulation component involved in a modulation signal at a desired characteristic frequency of the optical frequency division multiplexed signal.

An optical transmitter includes: a selection circuit; a signal processing circuit; an optical modulator; and a control circuit. The selection circuit selects signal components on the sides of the maximum and minimum values in a multivalued electrical signal for modulation of a transmitting signal. The signal processing circuit generates a multivalued electrical signal into which the transmitting signal is converted by a combination of a superimposed signal that a low-frequency wave is superimposed on the signal components and a signal having a plurality of intermediate amplitude values on which the low-frequency wave is not superimposed. The optical modulator modulates a carrier light on the basis of the multivalued electrical signal. The control circuit controls the reference amplitude value or amplitude of the multivalued electrical signal on the basis of the low-frequency components contained in a modulated optical signal.

A digital instruction (216) is generated regarding one or more electrical-to-optical conversion impairments induced at the transmitter (202) of an optical communication system (200). 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 (208) of the optical communication system to compensate for the impairments.

An all-optical modulator(1) includes a directional coupling unit(11) that multiplexes, on a carrier light beam, a signal light beam of a modulation signal that a carrier signal at a characteristic frequency is modulated with an information signal. The all-optical modulator includes a nonlinear medium(12) that cross-phase-modulates the carrier light beam with the multiplexed signal light beam, and frequency-multiplexes the information signal in the signal light beam on the carrier light beam to generate an optical frequency division multiplexed signal. The all-optical modulator includes a monitoring control unit(15) that controls a polarization control unit(19) that controls a polarization state of the signal light beam in a direction in which an intensity of a modulation component takes a maximum value, based on the intensity of the modulation component involved in a modulation signal at a desired characteristic frequency of the optical frequency division multiplexed signal.