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.

Joint estimation of the framer index and the frequency offset in a optical communication system are described among various other features. A transmitter can transmit data frames using pilot and framer symbols. A receiver can estimate the framer index and frequency offset using the pilot and framer symbols, and identify the beginning of a header portion of a data frame. The estimation can be performed to compensate for delays such as half-symbol delays and differential group delays. By identifying the beginning of the header portion of a data frame while compensating for certain delays, the receiver can synchronize, with less error, the data transmitted by the transmitter and the data it received.

Methods for estimating the Effective Modal Bandwidth (EMB) of laser optimized Multimode Fiber (MMF) at a specified wavelength, λ.sub.S, based on the measured EMB at a first reference measurement wavelength, λ.sub.M. In these methods the Differential Mode Delay (DMD) of a MMF is measured and the Effective Modal Bandwidth (EMB) is computed at a first measurement wavelength. By extracting signal features such as centroids, peak power, pulse widths, and skews, as described in this disclosure, the EMB can be estimated at a second specified wavelength with different degrees of accuracy. The first method estimates the EMB at the second specified wavelength based on measurements at the reference wavelength. The second method predicts if the EMB at the second specified wavelength is equal or greater than a specified bandwidth limit.

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.

The present disclosure provides signal management with unequal eye spacing by: determining a dispersion slope of a channel between a transmitter and a receiver based on a temperature of the transmitter and a wavelength used by the transmitter to transmit signals over the channel; determining maximum and minimum powers for transmission over the channel; assigning a plurality of rails to a corresponding plurality of power levels, wherein amplitude differences between adjacent rails of the plurality of rails are based on the dispersion slope and produce a first eye pattern with a first Ratio of Level Mismatch (RLM) less than one; encoding, by the transmitter, data onto a conditioned signal according to the plurality of rails; and transmitting the conditioned signal over the channel, so that the conditioned signal demonstrates a second eye pattern with a second RLM greater than the first RLM when received at the receiver.

A monitoring device includes a processor configured to compensate an electric field signal generated from an optical signal alternately for a chromatic dispersion and a nonlinear distortion in the optical signal in each of virtual sections of a transmission line, evaluate a quality of a compensated electric field signal, select the virtual sections sequentially, set a first compensation quantity of the chromatic dispersion according to a length of each of the virtual sections, search for a third compensation quantity of the nonlinear distortion for a selected virtual section when the quality satisfies a predetermined condition under an assumption that no nonlinear distortion is produced in other virtual sections, search for a second compensation quantity of the nonlinear distortion by setting an initial value of the second compensation quantity to the third compensation quantity, and monitor a power distribution of the optical signal based on the first and second compensation quantities.

A skew compensation system for a coherent optical communication network includes a transmitter modulator having a first driver input for receiving a first signal from a first channel, a second driver input for receiving a second signal from a second channel, a source input for receiving a continuous wave source signal, and a modulation output in communication with an optical transport medium of the network. The system further includes a tunable delay line disposed between the second channel and the second driver input for inserting a pre-determined training sequence onto the second signal prior to the second driver input, and a processor for determining a skew amount between the second signal at the second driver input and the first signal at the first driver input, calculating a pre-compensation value corresponding to the skew amount, and reducing the skew amount at the modulation output according to the pre-compensation value.

A receiver is configured to calculate a representation of a received signal conveying symbols at a frequency f.sub.S, the representation comprising a first frequency band and a second frequency band which are disjoint and have non-zero correlation. 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 Δ and comprised within the first frequency band, and a second term comprising a function of a phase difference between the representation at a second pair of frequencies separated by the gap Δ and comprised within the second frequency band, wherein the higher frequency of the first pair and the higher frequency of the second pair are separated by a gap G. An estimate of chromatic dispersion in the received signal is calculated based on the first term and the second term.

The disclosed systems, structures, and methods are directed to an optical communication system. The optical communication system comprising an optical node configured to receive a light signal, in which the light signal is received from a first optical path and a second optical path, and the optical node is further configured to determine a change in state of polarization (SOP) of the light signal received from the first optical path and the second optical path, a processing unit configured to determine if at least a portion of the first optical path and the second optical path has a/similar change in SOP.

[Problem] A signal distortion generated when a multi-level modulated optical signal is transmitted through an optical transmission path where optical amplifiers are scattered is suppressed and transmission quality is improved.

[Solution] An optical transmission system 20 includes Tx 21a to Tx 21n configured to transmit a multi-level modulated optical signal 32 to an optical fiber 25, optical amplifiers 26a to 26f configured to amplify the optical signal 32 transmitted through the optical fiber 25, the optical amplifiers 26a to 26f being scattered on the optical fiber 25, and Rx 24a to Rx 24n configured to receive the amplified optical signal 32 via the optical fiber 25. A pre-dispersion compensation unit 27 of each of the Tx 21a to Tx 21n performs pre-dispersion compensation on the transmitted optical signal 32, based on a pre-dispersion compensation ratio for determining a percentage of dispersion compensation to be performed in advance on a wavelength dispersion to be accumulated in the optical fiber 25, with respect to the dispersion compensation to narrow a bandwidth to be widened by the wavelength dispersion during transmission of the optical signal 32 through the optical fiber 25.