A method includes modulating a digital signal via pulse amplitude modulation (PAM) and applying Nyquist shaping to the digital signal to generate a filtered digital signal. The method also includes converting the filtered digital signal into an analog signal and transmitting the analog signal in an optical communication channel via a dense wavelength division multiplexing (DWDM) scheme.

The disclosed structures and methods are directed to a method for compensation of linear and nonlinear effects in optical fiber of a coherent optical signal transmitted through an optical link. The method comprises receiving a coherent optical signal having carriers; determining values of intensity vectors for each carrier; determining values of filtered intensity vectors for each carrier by filtering the values of the intensity vectors at frequencies lower than a cut-off frequency of a filter; determining nonlinear compensation coefficients for each carrier based on the filtered intensity vectors; and modifying the digital coherent optical signal based on the nonlinear compensation coefficients.

A digital processor (DP) is configured to obtain a temporal sequence of digital phase distortion measurements of a first optical signal received by a coherent optical receiver (COR) from an optical fiber link, where the first optical signal co-propagates with a second, power-modulated, optical signal in different frequency channels. The DP is configured to estimate a cross-correlation between the temporal sequence of digital measurements and a temporal sequence of powers of the second optical signal for a plurality of relative time shifts between the sequences, and to identify a location along the optical fiber link based on a magnitude of the cross-correlation exceeding a threshold for a particular time shift.

An optical signal transmission apparatus and a method are provided. The apparatus may include a set of optical transmitters, each optical transmitter configured for transmission of light signals at one or more wavelengths of light. The apparatus may include an output port configured for transmission of the light signals through a span of optical fiber. The apparatus may use an input port configured to receive other transmissions of light signals through the span of optical fiber. The apparatus may include a set of light detectors, each light detector configured to detect a time delay or phase characteristic associated with the one or more wavelengths of light. The apparatus may include a processor configured to determine a metric based on the detected time delay or phase characteristic.

A skew adjustment method and a digital coherent receiver which can achieve skew adjustment without using a fixed pattern for skew detection are provided. A digital coherent receiver (100) includes: a chromatic dispersion adder (103) that adds chromatic dispersion to the optical multiplexed signal; a skew adjuster (201) that sets a quantity of skew adjustment for each of the plurality of channel signals obtained by detecting the optical multiplexed signal; and a skew controller (204) that is configured to, while monitoring signal quality of a reception signal obtained from the plurality of channel signals skew-adjusted, search for a quantity of skew adjustment at which the signal quality is made better.

A network element includes a client port configured to receive a signal for transmission; a line port configured to transmit the signal to a far end via Optical Transport Network (OTN); circuitry configured to communicate one or more of a fault and a status associated with the signal to the far end via OTN overhead. The circuitry configured to communicate can be for the fault and utilizes one or more Tandem Connection Monitoring (TCM) layers in the OTN overhead. The circuitry configured to communicate can be for the status and utilizes one or more of Optical Data Unit (ODU) Performance Monitoring (PM) and one or more Tandem Connection Monitoring (TCM) layers.

A band division timing adjustment unit aligns timings of a plurality of signals, which are generated by dividing a received signal according to a plurality of frequency bands, in a time domain and combines the plurality of signals for which the timings have been aligned. A chromatic dispersion compensation unit compensates chromatic dispersion of an output signal of the band division timing adjustment unit for each of the plurality of frequency bands.

The disclosed structures and methods are directed to a method for compensation of linear and nonlinear effects in optical fiber of a coherent optical signal transmitted through an optical link. The method comprises receiving a coherent optical signal having carriers; determining values of intensity vectors for each carrier; determining values of filtered intensity vectors for each carrier by filtering the values of the intensity vectors at frequencies lower than a cut-off frequency of a filter; determining nonlinear compensation coefficients for each carrier based on the filtered intensity vectors; and modifying the digital coherent optical signal based on the nonlinear compensation coefficients.

A method includes modulating a digital signal via pulse amplitude modulation (PAM) and applying Nyquist shaping to the digital signal to generate a filtered digital signal. The method also includes converting the filtered digital signal into an analog signal and transmitting the analog signal in an optical communication channel via a dense wavelength division multiplexing (DWDM) scheme.

A receiver is configured to calculate a representation of a received signal conveying symbols at a frequency f.sub.S, the representation comprising non-zero components at frequencies of magnitudes exceeding f.sub.S/2. The receiver calculates a first term comprising a function of a phase difference between the representation at a first pair of frequencies separated by a gap and comprised within a first band of width 2 centered at f.sub.S/2, and a second term comprising a function of a phase difference between the representation at a second pair of frequencies separated by the gap and comprised within a second band of width 2 centered at f.sub.S/2, wherein <2, and wherein the higher frequency of the first pair and the higher frequency of the second pair are separated by the frequency f.sub.S. An estimate of chromatic dispersion in the received signal is calculated based on the first term and the second term.