Apparatus and method for digital signal constellation transformation are provided herein. In certain configurations, an integrated circuit includes an analog front-end that converts an analog signal vector representing an optical signal into a digital signal vector, and a digital signal processing circuit that processes the digital signal vector to recover data from the optical signal. The digital signal processing circuit generates signal data representing a signal constellation of the digital signal vector. The digital signal processing circuit includes an adaptive gain equalizer that compensates the signal data for distortion of the signal constellation arising from biasing errors of optical modulators used to transmit the optical signal.

An optical signal processing method and a coherent receiver, wherein an in-phase signal XI in a first polarization direction and an in-phase signal YI in a second polarization direction are added up to obtain a signal I; a quadrature signal XQ in the first polarization direction and a quadrature signal YQ in the second polarization direction are added up to obtain a signal Q; and quantization, combination, and digital signal processing are performed on the I and the Q. After summation, two signals need to be quantized. Therefore, a quantity of ADCs is reduced by half. In addition, because power consumption of a summation component is less than that of an ADS, power consumption of optical signal processing can be reduced. In addition, because there is a preset value, the summation may be performed after phase-inversion is performed on one analog signal, thereby avoiding a signal loss caused by the summation.

An optical data receiver includes optical hybrids, light detectors and a digital signal processor. Each optical hybrid outputs mixtures of a corresponding one of the polarization components of a received data-modulated optical carrier with reference light. Each light detector outputs digital measurements of the mixtures from a corresponding one of the optical hybrids. The digital signal processor identifies data symbols of a constellation having parts transmitted on both polarization components of the data-modulated optical carrier responsive to receipt of the digital measurements. The transmitted data-modulated optical carrier has about a same total light intensity in each modulation time slot thereof. Each data symbol is defined by in-phase and quadrature-phase electric field coordinates of both polarization components. Pairs of in-phase and quadrature-phase electric coordinates of each of the polarization components are on a preselected set of one or more concentric circles about an origin. The constellation has 4D dimensions, D being an integer.

In some embodiments, an apparatus includes a memory and a processor operatively coupled to the memory. The processor is configured to be operatively coupled to a first optical transponder and a second optical transponder. The processor is configured to receive, from the second optical transponder, a signal representing a skew value of an optical signal and a signal representing a bit-error-rate (BER) value of the optical signal. The skew value is associated with a skew between an in-phase component of the optical signal and a quadrature component of the optical signal. The processor is configured to determine, based on at least one of the skew value or the BER value, if a performance degradation of the first optical transponder satisfies a threshold. The processor is configured to send a control signal to the first optical transponder to adjust a pulse shaping or a data baud rate of the first optical transponder.

An optical transmitter includes a signal processor and transmission circuit. The signal processor modulates a first signal to generate a first modulated signal, determines a modulo amplitude that is larger than an amplitude of the first modulated signal, inserts a second modulated signal into the first modulated signal to generate a transmission signal, corrects a symbol of the transmission signal by using an amplitude of one or a plurality of previous symbols to generate a pre-equalized signal, and performs modulo calculation based on the modulo amplitude on the pre-equalized signal. The transmission circuit generates a modulated optical signal based on an output signal of the signal processor and transmits the modulated optical signal to a reception node. An amplitude of the second modulated signal is equal to the modulo amplitude.

A coherent detection method and device, and an optical transmission system are disclosed. The method may include: acquiring an optical transmission signal and a local oscillator signal, where a frequency value of the local oscillator signal differs from a frequency value of the optical transmission signal by a preset offset frequency value, the preset offset frequency value being smaller than a symbol rate value of the optical transmission signal; obtaining a coupled signal according to the optical transmission signal and the local oscillator signal; converting the coupled signal into an analog signal; sampling the analog signal according to a preset sampling frequency value and converting the analog signal into a plurality digital signals; and processing two digital signals among the plurality of digital signals so that a phase difference between the two digital signals to is 90 degrees.

A coherent optical receiver in which the channel equalizer and the clock-recovery circuit are connected in a nested-loop configuration, wherein the channel estimate generated by the equalizer is used to adjust the phase of the clock signal generated by the clock-recovery circuit. The channel equalizer can be implemented using a bank of time-domain or frequency-domain FIR filters. In an example embodiment, the clock-recovery circuit is configured to track the phase rotation corresponding to the equalized signals in a frequency-dependent manner; track the phase rotation in the channel equalizer either in a frequency-dependent manner or based on the mean signal delay therein; and adjust the phase of the clock signal based on an effective difference between these two phase rotations. The clock-recovery circuit enhances the clock tone by applying a Fourier transform to the squared absolute values of the equalized signals outputted by the channel equalizer.

A system and method are disclosed to characterize and correct for the effects of IQ skew and insertion loss in a coherent optical transmitter. The coherent optical transmitter receives a digital data signal including in-phase (I) and quadrature (Q) components and generates corresponding first and second dither signals. The first dither signal may be combined with the I component and the second dither signal may be combined with the Q component to generate I and Q combined signals, which may be converted into I and Q analog waveforms. An optical signal may be generated corresponding to the I and Q analog waveforms for transmission over an optical fiber. The IQ skew and/or insertion loss for the coherent optical transmitter may then be calculated based on the optical signal using the disclosed dither tone processing techniques in order to correct IQ skew and/or insertion loss impairment.

A quadrature oscillator includes a first oscillator that outputs a first differential signal, and a second oscillator that outputs a second differential signal having phases that are different from those of the first differential signal, wherein the first oscillator includes a first LC resonator having an inductor and a capacitor coupled in parallel, a first cross-coupled circuit having a first pair of cross-coupled transistors coupled to the first LC resonator, a first tail current source coupled to the first pair of transistors, first input differential pair transistors to which the second differential signal is to be input, and a first pair of harmonic resonators disposed in input sections of the first input differential pair transistors, the first pair of the harmonic resonators have a resonance frequency of an odd multiple of a resonance frequency of the first oscillator.

In a coherent optical receiver device, the dynamic range considerably decreases in the case of selectively receiving the optical multiplexed signals by means of the wavelength of the local oscillator light, therefore, a coherent optical receiver device according to an exemplary aspect of the invention includes a coherent optical receiver receiving optical multiplexed signals in a lump in which signal light is multiplexed; a variable optical attenuator; a local oscillator connected to the coherent optical receiver; and a first controller controlling the variable optical attenuator by means of a first control signal based on an output signal of the coherent optical receiver; wherein the coherent optical receiver includes a 90-degree hybrid circuit, a photoelectric converter, and an impedance conversion amplifier, and selectively detects the signal light interfering with local oscillation light output by the local oscillator out of the optical multiplexed signals; and the variable optical attenuator is disposed in the optical path of the optical multiplexed signals in a stage preceding the photoelectric converter, inputs the optical multiplexed signals, and outputs them to the coherent optical receiver controlling the intensity of the optical multiplexed signals based on the first control signal.