An I component compensation unit calculates an I component in which a distortion has been compensated, by forming a first polynomial expressing the distortion of the I component based on an I component and a Q component of a quadrature modulation signal and multiplying each term of the first polynomial by a first coefficient. A Q component compensation unit calculates a Q component in which a distortion has been compensated, by forming a second polynomial expressing the distortion of the Q component based on the I component and the Q component of the quadrature modulation signal and multiplying each term of the second polynomial by a second coefficient. A coefficient calculation unit calculates the first and second coefficients by comparing outputs of the I component compensation unit and the Q component compensation unit and a known signal.

There is provided a receiving device including a hardware processor configured to demodulate a signal into which a first signal and a second signal are wavelength-multiplexed, into a first baseband signal and a second baseband signal corresponding to the first signal and the second signal, respectively, extract, from the second baseband signal, a signal component of crosstalk from the second signal to the first signal, shift a frequency of the extracted signal component, and compensate for the crosstalk from the second signal to the first signal, based on the extracted signal component shifted by the frequency.

We disclose an optical transport system configured to reduce nonlinear signal distortions using an electronic phase rotation, the phase value of which is determined using pre-filtering, e.g., via a low-pass filter, of the digital samples representing an optical communication signal prior to applying a squaring operation to the digital samples. In some embodiments, the phase value used in the electronic phase rotation can be determined using double filtering of the digital samples that, in addition to the pre-filtering, employs post-filtering, e.g., via another low-pass filter, of the digital samples generated by the squaring operation. The electronic phase rotation can be implemented as part of a backward-propagation algorithm that, in addition to reducing the nonlinear signal distortions, provides at least partial dispersion compensation. In various embodiments, the corresponding backward-propagation module can be incorporated into the transmitter's digital signal processor (DSP) or the receiver's DSP.

Techniques and circuits are proposed to increase averaging in the clock recovery band based on an amount of channel overlap in receivers using excess bandwidth for clock recovery, to mitigate the impact of spectral energy leaking into an active channel of interest from an adjacent active channel and to improve the accuracy of the phase estimate of the received transmitted clock.

A coherent detection method, apparatus, and system are disclosed. The method may include: receiving an intensity-modulated optical signal transmitted by a transmitting end, where the intensity-modulated optical signal is obtained by intensity modulation performed by the transmitting end on an original signal; performing phase modulation on a local oscillator optical signal to obtain a phase-modulated local oscillator optical signal; and mixing the intensity-modulated optical signal and the phase-modulated local oscillator optical signal, and then performing photoelectric detection, analog-to-digital conversion, and digital signal receiving processing in sequence to recover the original signal.

A WDM receiver configured to apply electronic equalization processing to both dispersion-compensated and dispersion-distorted versions of the received communication signal. In an example embodiment, the receiver's DSP first generates an equalized dispersion-compensated signal corresponding to the communication signal. The DSP then performs electronic dispersion-application processing on the equalized dispersion-compensated signal to generate a dispersion-distorted version thereof. The DSP then applies decision-aided electronic equalization processing to the dispersion-distorted version of the signal, subjects the resulting equalized signal to another round of dispersion-compensation processing, and recovers the data encoded in the communication signal using the resulting dispersion-compensated signal. This chain of signal processing tends to be effective in reducing nonlinear distortions of the intra-channel type and also some effects of nonlinear inter-channel interference, which advantageously enables the WDM receiver to support a lower BER and/or a higher data-transport capacity than those achievable with conventional WDM receivers.

An optical transmitter apparatus is disclosed. The apparatus includes a processor, a memory coupled to the processor, and one or more programs configured to be executed by the processor. The programs include instructions for nonlinearity estimation that characterizes nonlinearity in an optical communication and estimates an amount of symbol distortion caused by the nonlinearity, instructions for selecting and mapping symbols to provide, for the nonlinearity estimation, only symbols that meet predetermined nonlinearity criteria, and instructions for storing, in the memory, the amount of symbol distortion to be used for a nonlinearity pre-compensation.

An I component compensation unit calculates an I component in which a distortion has been compensated, by forming a first polynomial expressing the distortion of the I component based on an I component and a Q component of a quadrature modulation signal and multiplying each term of the first polynomial by a first coefficient. A Q component compensation unit calculates a Q component in which a distortion has been compensated, by forming a second polynomial expressing the distortion of the Q component based on the I component and the Q component of the quadrature modulation signal and multiplying each term of the second polynomial by a second coefficient. A coefficient calculation unit calculates the first and second coefficients by comparing outputs of the I component compensation unit and the Q component compensation unit and a known signal.

A system has a plurality of non-linear circuit stages and an intervening linear circuit stage. An input signal is provided to a first non-linear circuit stage, and from the first non-linear circuit stage, to the linear circuit stage. The first non-linear circuit stage applies a second-order distortion to the input signal and provides the resulting signal to the linear circuit stage. The resulting signal that is output from the linear circuit stage is inverted with respect to the input signal and suitably linearly processed (attenuated or amplified). This signal is then provided to a second non-linear circuit that applies a second-order distortion and outputs a signal that has an overall reduction in second-order distortion.

Techniques and circuits are proposed to increase averaging in the clock recovery band based on an amount of channel overlap in receivers using excess bandwidth for clock recovery, to mitigate the impact of spectral energy leaking into an active channel of interest from an adjacent active channel and to improve the accuracy of the phase estimate of the received transmitted clock.