An estimating device includes a detector and an estimator. The detector detects a first parameter representing a ratio of a power of a signal component in received light to a power of a noise component in the received signal, a second parameter representing a sum of the power of the signal component and the power of the noise component, and a third parameter representing a sum of a power of a component being derived from cross phase modulation and being contained in the noise component and a power of a component being derived from spontaneously emitted light and being contained in the noise component. The estimator estimates, based on the first through the third parameters, a fourth parameter representing a ratio of the power of the signal component to a power of a component being derived from self phase modulation and being contained in the noise component.

The present invention proposes a method for monitoring the nonlinear effect of an optical fiber link by fractional Fourier transformation, FRFT, by calculating an optimal fractional order of the FRFT of the frequency-domain signal propagating through an optical fiber link, calculating the chromatic dispersion of an optical fiber link based on the optimal fractional order, compensating for chromatic dispersion to the signal, calculating an optimal fractional order of the FRFT for the time-domain signal following the compensation for chromatic dispersion, calculating the time-domain chirp caused by the nonlinear effect of an optical fiber link based on the optimal fractional order, and monitoring the nonlinear effect of an optical fiber link based on the absolute value of the calculated time-domain chirp. The method can be used for quantitatively monitoring the nonlinear effect of an optical fiber link in an optical fiber communication system consisting of different types of optical fibers.

A method to capture random data signals at an end point in a broadband network and process them via digital signal processing (DSP) techniques to determine both linear distortions and nonlinear distortions. In a distribution network, such as a tree and branch cable network, the location of the impairment addition can be identified by determining location of terminals have a distortion and locations of terminals that do not have a distortion. Linear distortions may be determined by an autocorrelation of the captured signal with itself. Nonlinear distortions may be determined by processing measured energy in a vacant band with manufactured energy in the vacant band. If a vacant band is not available, one can be created by demodulating a signal occupying the band, and subtracting the demodulated signal from the measured signal plus interference in a band, leaving only the interference.

Provided an optical transmission characteristic estimation device that models an optical effect of an optical transmission line by a nonlinear Schrodinger equation, divides the optical transmission line into N (N is an integer of two or more) sections, obtains a solution of the nonlinear Schrodinger equation by an operation of N steps in which a section length of each of the divided N sections is a step size, and estimates transmission characteristics of the optical transmission line, the optical transmission characteristic estimation device including a step size update unit configured to update each of the N step sizes on the basis of an estimated optical power distribution and an estimated dispersion distribution, which are indicated by the transmission characteristics.

A method for monitoring generalized optical signal-to-noise ratio (gOSNR) is provided, which is applied to monitoring of an optical signal received by a coherent receiver through a fiber link. The method includes: obtaining a first received waveform; obtaining a signal part of the first received waveform; obtaining a noise part of the first received waveform according to the signal part and the first received waveform; obtaining a first correlation between the noise part and a first template at a predetermined location on the fiber link, the first correlation indicating a signal power at the predetermined location; obtaining a second correlation between the noise part and a second template at the predetermined location on the fiber link, the second correlation indicating a signal power and noise power at the predetermined location; and obtaining gOSNR at the predetermined location according to the first correlation and the second correlation.

A nonlinearity compensation technique for a CO-OFDM transmission system in which a proportion (e.g. up to 50%) of OFDM subcarriers is transmitted along with a phase-conjugate copy (PCP) on another subcarrier (replacing a data carrying subcarrier) to enable nonlinear distortion compensation. Nonlinear distortion experienced by closely spaced subcarriers in an OFDM system is highly correlated. The PCPs are used at the receiver to estimate the nonlinear distortion (e.g. nonlinear phase shift) of their respective original subcarriers and other subcarriers close to the PCP. With this technique, the optical fibre nonlinearity due to the Kerr effect in OFDM systems can be effectively compensated without the complexity of DBP or 50% loss in capacity of the phase conjugate twin wave (PC-TW) technique. Moreover, the technique proposed herein can be effectively implemented in both single polarization and PMD systems, in both single channel and WDM systems.

A system and method for mitigating nonlinearity in an optical communication link with multiple carriers uses mutual frequency referencing to stabilize at least a portion of the multiple carriers. Using at least one frequency-referenced signal, carrier nonlinearity can be determined and compensated within the link by pre-distortion, back-propagation, or a combination of both. Mutual frequency referencing may be performed at the emitting end of the link, at the receiving end, or a combination of both.

The present invention proposes a method for monitoring the nonlinear effect of an optical fiber link by fractional Fourier transformation, FRFT, by calculating an optimal fractional order of the FRFT of the frequency-domain signal propagating through an optical fiber link, calculating the chromatic dispersion of an optical fiber link based on the optimal fractional order, compensating for chromatic dispersion to the signal, calculating an optimal fractional order of the FRFT for the time-domain signal following the compensation for chromatic dispersion, calculating the time-domain chirp caused by the nonlinear effect of an optical fiber link based on the optimal fractional order, and monitoring the nonlinear effect of an optical fiber link based on the absolute value of the calculated time-domain chirp. The method can be used for quantitatively monitoring the nonlinear effect of an optical fiber link in an optical fiber communication system consisting of different types of optical fibers.

The invention relates to an optical spectrum inverter, configured for counteracting phase distortion effects in an optical channel over a predefined frequency range, to an inverter node, configured for duplex operation in at least two wavelength channels, and to a method for counteracting phase distortion effects in an optical channel. The optical spectrum inverter comprises an optical receiver comprising a polarization diversity unit configured for polarization resolved detection, wherein the optical receiver is configured for receiving an optical input signal comprising a first wavelength span and for converting the optical input signal to an electrical signal, and an optical modulator connectable to the optical receiver and configured for modulating the electrical signal onto an optical signal, wherein the optical modulator comprises an optical filter which is configured for selecting a second wavelength span of the optical signal corresponding to an optical output side band of an inverted spectrum of the optical channel such that phase distortion effects are counteracted at least partly over the predefined frequency range. In this way, an optical spectrum inverter is provided which is simple and cost-effective to realize and allows higher optical power levels in conjunction with a higher capacity per optical channel and is also adapted for longer transmission distances.

A nonlinearity compensation circuit includes a detector that detects a combination of input levels of a plurality of input signals, a memory that saves a correction value information item to compensate for nonlinear distortion, the correction value information item being saved corresponding to the combination of the input levels, and a compensator that corrects a target signal among said plurality of input signals using the correction value information item acquired from the memory based on the combination of the input levels detected by the detector.